s///|s()()i|/i|/g|\U|\u|\L|\l|\U\l|split|join|匹配到hash|匹配到变量|`date`|$^I

 #!/usr/bin/perl -w
 
 use strict;
 use warnings;
 
 $_='She is a good girl
 and  likes helping others
 ';
 
 s/she/he/i;print "$_";
 
 #he is a good girl
 #and  likes helping others
 
 s/hae/she/i;print "$_";
 
 #he is a good girl
 #and  likes helping others
 
 s/(he)/$@ @/i;print;
 
 #he@ @ is a good girl
 #and  likes helping others
 
 s/s$/ours,/m;print;
 
 #he@ @ is a good girl
 #and  likes helping otherours,
 
 s/(\w*)ing/$/g;print;
 
 #he@ @ is a good girl
 #and  likes help otherours,
 
 $_ ='homeN home ome ooo
 ';
 
 s/h/i/;print;
 
 #iomeN home ome ooo
 
 s/o/g/g;print;
 
 #igmeN hgme gme ggg
 
 s(gme)()g;print;
 
 #i123N h123 123 ggg
 
 s(n)(pppp)i;print;
 
 #i123pppp h123 123 ggg
 
 (my $copy = $_) =~ s/\s/__/g;print "$copy$_";
 
 #i123pppp h12__12__ggg
 #i123pppp h123 123 ggg
 
 #my $copy = $_ =~ s/3\s/__/g;print "$copy$_";
 
 #i123pppp h123 123 ggg
 #2i123pppp h12__12__ggg
 
 s/(\w*)/\U$/gi;print;
 
 #I123PPPP H123 123 GGG
 
 s/(\w*)\s(\w*)/\l$/i;print;
 
 #i123PPPP 123 GGG
 
 s/(GGG)/\u\L$/gi;print;
 
 #i123PPPP 123 spGgg
 
 print "\u\Lmy name is \Uyy\n";
 
 #My name is YY
 
 my @split = split / /,$_;print "@split";
 
 #i123PPPP 123 Ggg
 
 my @new_split = split;print"@new_split\n";
 
 #123PPPP 123 Ggg
 
 my @new_split1 = split /\s/;print"@new_split1\n";
 
 #123PPPP 123 Ggg
 
 my $glue = 'xxx';my $string = join $glue,@new_split;print "$string\n";
 
 #i123PPPPxxx123xxxGgg
 
 $_ = 'aaa bb ddd';
 my ($next1,$next2,$next3) = /(\S+) (\S+) (\S+)/;print "$next2\n";
 
 #bb
 
 my %hash = ($string =~ /([A-Z]+)([-]+)/gi);
 
 foreach my $key(keys %hash)
 {
     print "$key and $hash{$key}\n";
 }
 
 #PPPPxxx and 123
 #i and 123
 
 $_ = "a\nmmmmmmm\nabc\n\s\s\ssdsfs\n";
 print "$1\n" if /(.*c$)/gm;
 
 #abc
 
 my $time = `date`;print "$time";
 
 #Sun Jun 23 21:34:46 HKT 2019
 
 $^I = ".bak";
 
 my $input = <>;

问题：  cat .txt |perl test.pl 后并无备份文件1.txt.bak？