use strict;
use warnings;

my %info;
my $thisuser;

while (<DATA>) {
  my ($var, $val) = /<([^>]+)>([^<]+)/;
  if ($var eq 'UserID') {
    $thisuser = $val;
  }
  else {
    $info{$thisuser}{$var} = $val;
  }
}

use Data::Dumper;
print Dumper(\%info), "\n";

__DATA__
<UserID>46786<UserID>
<start>2004-10-21TO09:57:25Z</start>
<dev>Some Text</dev>
<var1>some string</var1>
<var2>some string</var2>
<USerID>57864</UserID>
<start>2004-10-25TO09:57:25Z</start>
<dev>Some Text</dev>
<var1>some string</var1>
<UserID>46786<UserID>
<var3>some string</var3>
<var4>some string</var4>
<UserID>98766</UserID>
<start>2004-10-21TO09:57:25Z</start>
<dev>Some Text</dev>
<var1>some string</var1>
<var2>some string</var2>
<var5>some string</var5>
<var6>some string</var6>
<USerID>57864</UserID>
<var4>some string</var4>
<var6>some string</var6>