The solution: don't do that:
    my $broken_links = ( defined $_[1] ) ? $_[1] : return ;

In addition, my own personal preference is to separate argument assignment and validation within a function rather than trying to bury them both in a single statement. I feel it leads to more readable and maintainable code.

sub find_broken_links {

    my ($location,
        $broken_links,
        ) = @_;

    return unless defined($location) and defined($broken_links);

    ... do stuff ...

    push @$broken_links, 'stuff';

    return;

    }
[download]

#!/usr/bin/perl -w
use strict;

my @broken = (1,2,3);
my $location = "/home/user/";
find_broken_links( $location, \@broken );
foreach my $var (@broken)
{
    print "Returned: $var\n";
}


sub find_broken_links
{
  my ($location, $broken_links) = @_;
  # other code not shown here
  my $var ='X';
  push @$broken_links, $var;
}

#prints:
#Returned: 1
#Returned: 2
#Returned: 3
#Returned: X
[download]

perllexwarn explains why use warnings is better than -w. use warnings FATAL => 'all' is even better than use warnings. And if you expect warnings to do parameter validation for you, you should be extra careful. It's usually easier to check them.

This reference to perllexwar basically says that the problem can be that you turn warnings ON for more than maybe you want to.

I figure that more aggressive warnings are usually better than less aggressive. If you get warnings that you decide are not warranted, then you can "crank down the warning level" for that segment of code, but usually (in my opinion) that's the wrong thing to do unless you are extremely clear about why you are going that.

As far as parm validation goes, there is a whole bunch that didn't get talked about for this code example....use of prototypes in Perl functions, etc. In this case the issue was defined or not which is different than say param #3 should be between 4 and 12.

In any event, I think that it is wrong to return silently when the sub fails.

Update: In any event, I think that it is wrong to silently return a valid return value when the sub fails.