As other people have said - i would set the value to nothing at the very start of the loop. Then initialise it to the input value after that so its a clean value each time. BUT, would set the variable after the condition is true:

sub mehtod($)
{
    my $y = @_;
    my $x = "";  # or    
    my $x = 0;   # then    
    if(/expression/)
    {
      $x = $y;
    }
}
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Thats my preferable way of doing thins though, which does exactly the same as yours but means 3 extra line of code - ooooooh!