#! /usr/bin/perl
use strict;
use warnings;

sub regex {
    my ( $in  ) = @_;

    $in =~ m/^(a)(?{$_[ 1 ] = 1})/;
}

while(<DATA>) {
    my $placeholder;
    &regex( $_, $placeholder );
    print $placeholder . "." if $placeholder;
}

__DATA__
a
b
a
abcd
bcda