The error message tells you

Modification of a read-only value attempted

So re-write it:

#!/bin/perl
use strict;
use warnings;

my @vals = ( 5, 6, 7 );

my @area = &circ_area( @vals );
foreach (@area)
{
          print $_ . "\n";
}

sub circ_area
{
    foreach (@_)
    {
         $_= 3.14 * ($_**2);
    }
    return (@_)
}
[download]

But since you're modifying the @_ var ... you really don't need to return it ... you're just needlessly copying the data.

#!/bin/perl
use strict;
use warnings;

my @vals = ( 5, 6, 7 );

&circ_area( @vals );
foreach (@vals)
{
          print $_ . "\n";
}

sub circ_area
{
    foreach (@_)
    {
         $_= 3.14 * ($_**2);
    }
}
[download]

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