use warnings;
use strict;
use PDL;

my @funcs = (
    '1/(0.5 + 0.02 * $i*$i) * cos($i)',
    '1/(0.5 + 0.02 * $i*$i) * cos($i)',
);
my $surface = initPDL (\&shape, (10) x @funcs);

...

sub shape {
    my @values = @_;
    my $sum = 0;

    for my $idx (0 .. $#values) {
        my $i = $values[$idx];
        my $value = eval $funcs[$idx];

        $sum += $value * $value;
    }

    return sqrt ($sum);
}