how about this sollution

open FH1, "</users/mhalder/testrun/datafile";

foreach $line (<FH1>){
    
    $line=~s/\s+/:/g;
    ($fileid,$tag)=split (/\:/,$line);

    if (defined $filelist->{$tag}){

         $filelist->{$tag}->{COUNT}++;
         $filelist->{$tag}->{FILES}->[($filelist->{$tag}->{COUNT})-1]=
+$fileid;
     }
    else{
        $filelist->{$tag}->{COUNT}=1;
        $filelist->{$tag}->{FILES}->[($filelist->{$tag}->{COUNT})-1]=$
+fileid;

    }

}

print " **** OUTPUT **** \n\n";

foreach $filetag ( sort keys %$filelist){

    print "\n$filetag: $filelist->{$filetag}->{'COUNT'} FILES\n\n";
     print "LIST OF FILES:\n";
 
     foreach $filename ( @{$filelist->{$filetag}->{FILES}}){
           print " $filename\n";
     }
 
     print;
 
 
}
[download]

in my code "datafile" holds the lines 1300018 a1
1300019 a1
1300100 a2
1300101 a2
1300126 a2
1300127 a2
1300128 a2
1300129 a2
1300130 a3
1300131 a3
1300132 a3
1300133 a3