That's because after the replacement, the pattern no longer matches the string. 
#! /usr/bin/perl
use warnings;
use strict;

my $input = 'a';

my $doesnt_work = $input;

for (1 .. 10) {
    my $works = $input;

    $works       =~ s/a/$_/;
    $doesnt_work =~ s/a/$_/;

    print "Works: $works\n", "Doesn't work: $doesnt_work\n";
}

[download]
($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re: s// doesn't work when second loop! by choroba
in thread s// doesn't work when second loop! by SoulRule

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