Use File::Tie:

#!perl

use strict;
use warnings;

use Tie::File;

my $dir = q[path/to/dir];

opendir(DIR, $dir) or die "Could not open '$dir': $!\n";
my @files = grep { -T and /a\d+\.txt/ } readdir(DIR);
close DIR;

for my $file (@files) {
   my @lines;
   tie @lines, 'Tie::File', "$dir/$file" or die "Failed tieing '$file'
+: $!\n";
   unshift @lines, $file;
   untie @lines;
}
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Cheers,
CombatSquirrel.

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Entropy is the tendency of everything going to hell.