Your program doesn't quite work as you've presented it:

1. You haven't specified the name of the file "$pfile".
2. There's a "Bareword" error in $x=x;
3. You haven't supplied the subroutine trim

Additionally, I'd highly recommend you use strict and warnings at the top of the program.  This will make it necessary to declare your variables before you use them.  For example:

use strict;
use warnings;

# Global variables
my $cnt = 0;
my $header;
my $sub_line;
my $prev_index;
my $x;
my $c;
my $prev_user;
my $prev_id;
my $period;
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Finally, it would be a help if you could provide an example of what output you'd like to see as well.