@array = qw/P H A G E P H A G E P Q K R E P H A G E P W S Q E P H A G E P R D L E P H A G E/;

##</code><code>##

my $start = 0;
my $end = 4;
while ( $end <= $#array ) {
    my $string = join( "", @array[$start..$end];
    if ( $string eq 'PHAGE' ) {
        print "$string found in elements $start .. $end\n";
    }
    $start += 5;
    $end += 5;
}