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use strict;
use warnings;
...
$repl: --ab
old $str: abcd
new $str: --abcd

my $str = 'abcd';
my $pattern = shift;
...
$repl: --ab
old $str: abcd
new $str: --abcd

my $str = 'abcd';
my $pattern = shift;
...
$repl: --$1
old $str: abcd
new $str: --$1cd

my $str = 'abcd';
my $pattern = shift;
...
old $str: abcd
Use of uninitialized value $repl in substitution (s///) at 2perl.pl li
+ne 42.
new $str: cd