#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

my $string = "12345";
$string = substr( $string, 1 ) . substr( $string, 0, 1 );
say $string;

__END__
23451

##</code><code>##

$ perl -WE '$string = "12345"; $string =~ s/(.)(?=(.))/$2$1/g; say $string'
213243545