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use strict;
use warnings;

my $seq = "MTVLLLVVLQMWKATAGHSIAVSQDDGADDWETDPDFVNDVSEKEQRWGAKTVKGSGHQE";

my $peptide = getPeptide($seq,2,7,7);
print $peptide;

sub getPeptide {
	my ($seq, $site, $left, $right) = @_;
	my $len = length($seq);
	die " > seq length: $len\n" if $len < $site;
	die "'$site' < 1\n" if $site < 1;	
	$seq = ("*" x $left).$seq.("*" x $right);
	my $peptide = substr($seq, $site - 1, $right + $left + 1);
	return $peptide;
}