Why are there several c files in symbol table of %main::?

Question

'_<perlmain.c' => *{'::_<perlmain.c'},
'_</usr/lib64/perl5/5.8.8/x86_64-linux-thread-multi/auto/Data/Dumper/Dumper.so' => *{'::_</usr/lib64/perl5/5.8.8/x86_64-linux-thread-multi/auto/Data/Dumper/Dumper.so'},
'_<universal.c' => *{'::_<universal.c'},
'_<xsutils.c' => *{'::_<xsutils.c'},
...

Why are they in the symbol table of %main::,when are they useful?

Answer 1

To repeat the output from the question, run

#! /usr/bin/env perl
use Data::Dumper;
print Dumper \%main::;

The entries you see are inserted in gv_fetchfile_flags:

/* This is where the debuggers %{"::_<$filename"} hash is created */
tmpbuf[0] = '_';
tmpbuf[1] = '<';
memcpy(tmpbuf + 2, name, namelen);
gv = *(GV**)hv_fetch(PL_defstash, tmpbuf, tmplen, TRUE);
if (!isGV(gv)) {
    gv_init(gv, PL_defstash, tmpbuf, tmplen, FALSE);
#ifdef PERL_DONT_CREATE_GVSV
    GvSV(gv) = newSVpvn(name, namelen);
#else
    sv_setpvn(GvSV(gv), name, namelen);
#endif
}

This is called many times by way of newXS as part of the boot process in S_parse_body.

boot_core_PerlIO();
boot_core_UNIVERSAL();
boot_core_mro();

Note that you also see entries for perlio.c, universal.c, and mro.c in the output.

The Debugger Internals section of the perldebguts documentation explains their use:

For example, whenever you call Perl's built-in caller function from the package DB, the arguments that the corresponding stack frame was called with are copied to the @DB::args array. These mechanisms are enabled by calling Perl with the -d switch. Specifically, the following additional features are enabled (cf. $^P in perlvar):

• …
• Each array @{"_<$filename"} holds the lines of $filename for a file compiled by Perl. The same is also true for evaled strings that contain subroutines, or which are currently being executed. The $filename for evaled strings looks like (eval 34). Code assertions in regexes look like (re_eval 19).
• Each hash %{"_<$filename"} contains breakpoints and actions keyed by line number. Individual entries (as opposed to the whole hash) are settable. Perl only cares about Boolean true here, although the values used by perl5db.pl have the form "$break_condition\0$action".
  The same holds for evaluated strings that contain subroutines, or which are currently being executed. The $filename for evaled strings looks like (eval 34) or (re_eval 19) .
• Each scalar ${"_<$filename"} contains "_<$filename". This is also the case for evaluated strings that contain subroutines, or which are currently being executed. The $filename for evaled strings looks like (eval 34) or (re_eval 19).
• After each required file is compiled, but before it is executed, DB::postponed(*{"_<$filename"}) is called if the subroutine DB::postponed exists. Here, the $filename is the expanded name of the required file, as found in the values of %INC.
• …

Answer 2

As perl is a interpreter based language, it needs, right, his interpreter, the perl binary. This binary just reads the perl script, and executes the code by translating it in machine code.

Your perl interpreter is compiled with debug symbols, so it contains informations about the source files it build of. Also you see the objects of loaded modules Data::Dumper in your example.

Hope that helps