>perl -wMstrict -le
"my $s = '';
 if ($s != '') { print 'not equal' }
"
Argument "" isn't numeric in numeric ne (!=) at -e line 1.
Argument "" isn't numeric in numeric ne (!=) at -e line 1.

>perl -wMstrict -le
"my $s = 'foo';
 if ($s != '') { print 'not equal' }
"
Argument "" isn't numeric in numeric ne (!=) at -e line 1.
Argument "foo" isn't numeric in numeric ne (!=) at -e line 1.

>perl -wMstrict -le
"my $s = 0;
 if ($s != '') { print 'not equal' }
"
Argument "" isn't numeric in numeric ne (!=) at -e line 1.

>perl -wMstrict -le
"my $s = [ 'array', 'ref' ];
 if ($s != '') { print 'not equal' }
"
Argument "" isn't numeric in numeric ne (!=) at -e line 1.
not equal