perl的unless -d是什么意思？

#!/usr/bin/perl -w use strict; die "perl $0 " unless(@ARGV == 1); open IN,$ARGV[0]; open OUT,">new.txt"; while(){ chomp; my $cntT = ($_=~s/T/T/gi); my $cntA = ($_=~s/A/A/gi); my $cntCG = ($_=~s/CG/CG/gi); print "$_\t",sprintf("...