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

my $logfiles = 'C:\test\xunix\logfile_tracefiles\TraceLog_.txt';
my $seek = qr/UserName(.+)/;

$/="\x0A"; # gemeinsamer Nenner für Windows/Linux/Unix
open(my $fh, '<', $logfiles) or die "Can't open $logfiles ($!)\n";
while(my $line=<$fh>)
{
  if($line=~$seek)
  {
    print $1;
  }
}
close($fh);