The problem is that ! does not provide a numeric context.

use Scalar::Util qw[ dualvar ];;

sub alwaysFails { return dualvar 0, 'Some text to explain the error'; 
+};;

$rv = alwaysFails();;

print "Sub failed with error: $rv" if !$rv;;
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If you want 0 to stand for failed, you'd need to use a test that does provide a numeric context. Eg.

print "Sub failed with error: $rv" if ! 0+$rv;;
Sub failed with error: Some text to explain the error

print "Sub failed with error: $rv" if ~$rv;;
Sub failed with error: Some text to explain the error

print "Sub failed with error: $rv" if $rv == 0;;
Sub failed with error: Some text to explain the error
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