You're trying to modify read-only values (as Perl has no doubt told you).

Inside your subroutine, @_ contains three values (5, 6 and 7). But they aren't just values, they are the constants 5, 6 and seven. And in the foreach loop, $_ is aliases to each of those constants in turn. So when you try to assign a new value to $_, you are trying to overwrite the value of a constant. Which Perl, very sensibly, prevents you from doing.

What you really want is something like this:

sub circ_area {
  my @radii = @_; # copies values into a variable

  foreach (@radii) {
    $_= 3.14 * ($_**2);
  }

  return @radii;
}
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Or, more Perlishly:

sub circ_area {
  return map { 3.14 * ($_**2) } @_;
}
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