Run this code and you'll see exactly why that code is not helpful. It's not about the regex, it's about the double loop.

#!/usr/bin/perl -w

my @animals = ( "bear<1>\n", "camel<0> <2>\n", "<2>horse\n", "duck<3>\
+n" );
my @changeTo = ( 'A', 'B', 'C', 'D' );
my @toFind = ( '<0>', '<1>', '<2>', '<3>' );

@hs{@toFind}= (@changeTo);

$i = 0;
while($i < 5) {
        foreach $anm (@animals) {
                $anm =~ s/$_/$hs{$_}/g, for(keys %hs);
                print "$anm";
        }
        shift(@changeTo);
        push(@changeTo, int(rand(10000)));
        @hs{@toFind} = (@changeTo);
        print "\n";
        print "@changeTo";
        $i++;
        print "\n\n";
}
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