use constant UNPACKER => 'A24 A*';

open IN, '<', \$file1;
while (<IN>) {
    # my ($key, $tail) = m/(.{24})(.*)/;
    # my ($key, $tail) = unpack('A24 A*', $_);
    # my ($key, $tail) = unpacker();
    my ($key, $tail) = unpack(UNPACKER, $_);

    push @{$f1Lines{$key}}, [$tail, $.];
}
close IN;

open IN, '<', \$file2;
while (<IN>) {
    # my ($key, $tail) = m/(.{24})(.*)/;
    # my ($key, $tail) = unpack('A24 A*', $_);
    # my ($key, $tail) = unpacker();
    my ($key, $tail) = unpack(UNPACKER, $_);

    next unless exists $f1Lines{$key};

    my @matches = @{$f1Lines{$key}};

    print "Line $. of file2 ($key$tail) matches:\n";
    print "   line $_->[1] of file1 ($key$_->[0])\n" for @matches;
}
close IN;

sub unpacker { unpack('A24 A*', $_) }