use strict;
use warnings;
use 5.010;

my $line = 'hello world';

my ($subfamily) =  $line =~ /goodbye/;
#no match, and list context on the left, so m// returns ()

say "-->$subfamily<--";

if ($subfamily =~ /x/) {
    say 'yes';
}

--output:--
Use of uninitialized value $subfamily in concatenation (.) or string a
+t perl.pl line 10.
--><--
Use of uninitialized value $subfamily in pattern match (m//) at perl.p
+l line 11.
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Whenever you do a match, you need to check whether it succeeded or not. You never checked whether the first $subfamily match was true--like you did with the second match.