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