#!/usr/bin/perl -w
use strict;

my $str  = 'abcadefaghi';
my $pat  = '(a.)';
my $repl = '$1 ';

#$str =~ s/$pat/$1 /g;   # Working non-dynamic demo
$str =~ s/$pat/$repl/g;

print "$str\n";          # prints: '$1 c$1 ef$1 hi'