This works.

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

my $str  = 'abcadefaghi';
my $pat  = '(a.)';
my $repl = sub {"$1 "};

$str =~ s/$pat/&$repl/eg;

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