What is the difference between uppercase "-E" switch and lowercase "-e" in perl?

Question

How does the perl switches "-E" and "-e" differ from each other? In this example they works exactly the same — executes the command after the switch:

$ perl -e 'print "$_\n" foreach 1..2'
1
2
$ perl -E 'print "$_\n" foreach 1..2'
1
2

Answer 1

This is explained in perldoc perlrun:

-E commandline
behaves just like -e, except that it implicitly enables all optional features (in the main compilation unit). See feature.

The "See feature." refers to the documentation for the feature pragma, which you can read by typing perldoc feature.

Answer 2

-E unlike -e enables features

You can check what these are using Deparse module (following is for perl 5.16),

perl -MO=Deparse -E 1
use feature 'current_sub', 'evalbytes', 'fc', 'say', 'state', 'switch', 'unicode_strings', 'unicode_eval';

Answer 3

From Perldoc:http://perldoc.perl.org/perlrun.html

•-e commandline :

may be used to enter one line of program. If -e is given, Perl will not look for a filename in the argument list. Multiple -e commands may be given to build up a multi-line script. Make sure to use semicolons where you would in a normal program.

•-E commandline :

behaves just like -e, except that it implicitly enables all optional features (in the main compilation unit). See feature.