use warnings;
use strict;

my $dna = 'xxxxxxpecbcbccrlxxxxxxpeeeerlxxxxxplxxxxxPeRLxxxx';
my $re  = qr/p.*?l/i;

while ( $dna =~ m[($re)]g ) {
    my $beg = index( $dna, $1 );
    my $len = length($1);
    print join " " => $1, $beg, $beg + ($len -1), $/; # updated
}

##</code><code>##

pecbcbccrl 6 15 
peeeerl 22 28 
pl 34 35 
PeRL 41 44