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？