This isn't especially elegant, but it'll do the job, as long as your files are all in one directory.

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

# open current dir, and read in all files
#   matching /^a\d+\.txt/ into the array @files:
#
opendir my $dir_handle, "." or die "can't open dir";
my @files = grep { /^a\d+\.txt/ } readdir $dir_handle;
closedir $dir_handle;

# For each of the files:
#
foreach my $filename ( @files ) {
    # open the file, and read its contents into the
    #    array @file_contents, then close the file:
    #
    open my $fh, "$filename" or die "can't open $filename to read";
    my @file_contents = <$fh>;
    close $fh;

    # now reopen the file (for writing), and write your
    #    desired output to it:
    #
    open $fh, ">$filename" or die "can't open $filename to write";
    print $fh ">$filename\n@file_contents\n";
    close $fh;
}
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