I think changing the sub to return $var in event of it being unable to complete its intended purpose would be the best option.

I don't like it but if changing the sub can't be done then this is the most consice method I could think of. 

use strict;
use warnings;

my $var = 'foo';

my $tmp;
$var = ( $tmp = return_false() ) ? $tmp : $var;

print "$var\n";

$var = ( $tmp = return_true() ) ? $tmp : $var;

print "$var\n";


sub return_true
{
    return 'bar';
}

sub return_false
{
    return;
}

[download]

In reply to Re: Variable assignment error checking by rnewsham
in thread Variable assignment error checking by nathaniels

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