#!/usr/bin/perl
use strict;
use warnings;

my $impossible = qr/([245680])/;
my $n = 5;

print "$_\n" for (2, 3, 5);
for ( 1 .. 3_157 ) {
    $n += 2;
    my $skip;
    if ( $n =~ $impossible ) {
        $skip = adjust( $1, $n );
        next if $skip && $n =~ $impossible;
    }
    print $n, "\n" if rot_prime( $n );
    next if $skip;
    $n += 4;
    if ( $n =~ $impossible ) {
        adjust( $1, $n );
        next if $n =~ $impossible;
    }
    print $n, "\n" if rot_prime( $n );
}
print "Last number checked : $n\n";

sub rot_prime {
    my $n = shift;
    return 0 if ! is_prime( $n );
    my @dig = split //, $n;
    for ( 1 .. $#dig ) {
        push @dig, shift @dig; 
        return 0 if ! is_prime( join '', @dig );
    }
    return 1;
}

sub adjust {
    my $dig = shift;
    return 0 if $dig == 5 && $_[0] =~ /5$/;
    my $new = $dig;
    $new += $dig == 5 ? 2 : 1;
    $_[0] =~ s/$dig(\d+)$/$new . '1' x length( $1 )/e;
    $_[0] = $_[0] - $_[0] % 3;
    $_[0] += not ($_[0] % 2) ? -1 : 2;
    return $_[0];
}

sub is_prime {
    my $num = shift;
    my ($div, $sqrt) = (5, int sqrt $num);
    while ( $div <= $sqrt ) { 
        return 0 if not $num % $div;
        $div += 2;
        return 0 if not $num % $div;
        $div += 4;
    }
    return 1;
}
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