perl -e 'if( undef eq "" ) { print "In\n" } else { print "Out\n" }'
In

perl -e 'my $tmp; if( $tmp eq "" ) { print "In\n" } else { print "Out\n" }'
In


perl -e 'my $tmp; if( $tmp ne "" ) { print "In\n" } else { print "Out\n" }'
Out

perl -e 'if( undef ne "" ) { print "In\n" } else { print "Out\n" }'
Out