#!/usr/bin/perl

use strict;
use warnings;

my $file = '/path/to/file.txt';
open(my $fh,"<",$file) or die $!;
while(my $line = <$fh>){
  if($line =~ /Called-Station-Id/){
    my $userid = (split(/\s+/,$line))[1];
    print $userid;
  }
}
close $fh;