Try this.

#!/usr/bin/perl

use strict;

my %record;
while (<DATA>) {
    my ($key, $val) = split /\s+/;
    push( @{ $record{$val} }, $key );
}

for my $i ( sort keys %record ) {
    print "\n\n$i " . scalar @{ $record{$i} }
      . "file\n" . join "\n", @{ $record{$i} };
}

__DATA__
1300018   a1
1300019   a1
1300100   a2
1300101   a2
1300126   a2
1300127   a2
1300128   a2
1300129   a2
1300130   a3
1300131   a3
1300132   a3
1300133   a3
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