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

my $logfile ="C:/test/xunix/logfile_tracefiles/TraceLog_.txt";

open( my $infh, '<', $logfile ) or die "open($logfile,ro) failed: $!\n";

my $seek = qr{<UserName>(.+?)</UserName>};

# direkt zeilenweise lesen; spart RAM (und die Zaehlervariable)
while ( my $line = <$infh> ) {
    if ( $line =~ $seek ) {
        print $1, "\n";
    }
}
close $infh;