How is it that chomp is able to change the value of a variable not passed by reference?

Question

chomp seems to be able to change the value of a variable not passed by reference; that is, the syntax is chomp $var instead of chomp \$var.

How is this possible? How can I imitate this behavior in a function?

chomp:

my $var="foo\n";
chomp $var;
print $var

mychomp:

my $var="foo\n";
mychomp(\$var);
print $var;

sub mychomp {
  my $ref=shift;
  $$ref=~s/\s+$//;
}

Answer 1

All Perl parameters are "passed by reference"; more accurately, the contents of @_ are aliases for the actual parameters

Observe

use strict;
use warnings;
use 5.010;

my $s = 'abc';

upper_case($s);

say $s;


sub upper_case {
  $_[0] =~ tr/a-z/A-Z/;
}

output

ABC

Note that calling this function with a data literal, such as

upper_case('def')

will generate the fatal error

Modification of a read-only value attempted