# Precompile all the patterns and store them into the key CompiledPattern
$_->{'CompiledPattern'} = qr/$_->{'Pattern'}/i for values %$users;

while ( my $line = <> )
{
    ...

    my $user = first { $line =~ $users->{$_}{'CompiledPattern'} }
               keys %$users;

    if ( $user )
    {
        warn "Great, we found pattern $user->{'Pattern'}!\n";
    }
    else
    {
        warn "$line didn't match any users.\n";
    }
}