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my @data = ( [1 .. 3], [4 .. 6], [7 .. 9] );

...
        $data[$i][3] = $norm;
    }
}

#!/usr/bin/perl
use warnings;
...
        $data[$i][3] = $norm;
    }
}

sub norm{
    for(@data){ # iterates down @data
...
        $$_[3] = $norm;
    }
}

#!/usr/bin/perl
use warnings;
...
        $$_[3] = $norm;
    }
}

sub norm{
    my $ar = shift;
...
        $$_[3]=(($$_[0])**2+($$_[1])**2)**(1/2); #calculates the norm
    }
}