my $user;
my %uservariables;
while(<DATA>){
    chomp;
    my ($key, $value) = m{^<([^>]+)>(.*)<}i or die $_;
    $key = lc $key; #to adjust for case differences
    if ($key eq "userid"){
        $user = $value;
        next;
    }
    $uservariables{$user}->{$key}=$value;
}

use Data::Dumper;
print Dumper(\%uservariables);

__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>