#!/usr/bin/perl
use strict;

my $string = 'YYSFTVMETDPVN[115]HMVGV'
           . 'ISVEGRPGLFWFN[115]ISGGDK';
my @pos;
my $test = 'N[115]';

for ( my $i = 0; $i < length( $string ); $i++ ) {
    push( @pos, $i )
      if ( substr( $string, $i, 6 ) eq $test );
}

@pos = map { $_ == 0 ? $pos[$_] : $pos[$_] - 5 } (0 .. $#pos );
print "Found N at positions: ". join( ', ', @pos ), "\n";