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Subroutines in Perl

Named subroutines (or procedures or methods) are blocks of code that can take scalar inputs and return outputs. They have the general form sub name { code; }.

In this example we use a subroutine to define a function.
```
sub fx { my $t=shift(); return 4*cos($t); }
```
The shift operator grabs the input to the subroutine, which is then stored in the local scalar variable $t. This named subroutine can be used later, for example, to create an array @xcoord of x-coordinates of points on the radius 4 circle centered at the origin.
```
foreach my $i (0..6) {
   $xcoord[$i] = fx($i);
}
```

In this example, we create a subroutine that returns the max of two numerical inputs.

sub max {
    $a = shift();
    $b = shift();
    if ($a >= $b) {
       return $a;
    } else {
       return $b;
    }
}
max(-1,5);

By default, the inputs to a subroutine are stored in the local array @_ and the scalars in this array can be accessed via $_[0], $_[1], etc. We could rewrite our max subroutine as follows.
```
sub max {
    if ($_[0] >= $_[1]) {
       return $_[0];
    } else {
       return $_[1];
    }
}
max(-1,5);
```

Subroutines in Perl

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