I gather than line 68 (where the error is reported to have occurred) is here:
while ( my ( $url, $text ) = each(%hrefs) ) {
if ( !defined( $text->[-1] ) ) {
print $fh_ERR "$url|_____NO_TEXT_____\n";
}
elsif ( $text->[-1] ne $success ) {
$notfound++;
if (defined($text->[0])) {
my @text = collapse(@$text); ### <--- line 68
print $fh_ERR "$url|", join( ",", @text ), "\n";
}
else {
print $fh_ERR "$url|\n";
}
}
$total++;
}
I'm stumped about the reference to "aelem" in the error message -- no clue what this might be referring to. Apart from that, when you mention "the usual runtime", do you mean that this script has "usually" worked prior to this failure?
If so, the question becomes: what was different about this run relative to previous runs (when it worked as intended)? More input data? Corrupted input data? (I don't see much in the way of checking for bad input... what would happen if a line in your "$base_file.links" does not contain a "|" (vertical bar) character?)
Probably not related to your problem, but you could replace the "collapse" function call with:
grep { defined } @$text
Also, I see you checking for hash elements with
if(defined($hash{$key})), and it might make more sense to use
if(exists($hash{$key})) instead.