If the aim is really to compute:

sub diag {
my $x = shift @_;
my $y = shift @_;

my $x_sq = $x * $x;
my $y_sq = $y * $y;
my $prod = $x_sq * $y_sq;
my $result = sqrt($prod);
return $result;
}
[download]

then it will be presumably much faster to calculate it this way:

sub diag {
my $x = shift @_;
my $y = shift @_;
return abs ($x * $y);
}
[download]

or this way:

sub diag {
return abs ($_[0] * $_[1]);
}
[download]

Update: I've just corrected the code above adding the abs function which I had omitted, since my original function would have returned a negative number if one of the arguments were negative. Thanks to clueless newbie and davido who pointed that to me.