I hope this helps.

#!/usr/bin/perl -w

use strict;
use warnings;

my %MWs;

%MWs = ( "A" => 71.1,
         "D" => 115.0,
         "F" => 147.1,
         "H" => 137.1,
         "K" => 128.1,
         "M" => 131.1,
         "P" => 97.1,
         "R" => 156.1,
         "T" => 101.1,
         "W" => 186.2,
         "C" => 103.1,
         "E" => 129.1,
         "G" => 57.1,
         "I" => 113.1,
         "L" => 113.1,
         "N" => 114.1,
         "Q" => 128.1,
         "S" => 87.1,
         "V" => 99.1,
         "Y" => 163.1,
         "\n" => 0.0 );

my $sum = 0.0;

print "Type amino acids.\n";

while ( my $line = <STDIN> ) {
  chomp $line;
  last if $line eq '';
  my @AA = split '', $line;
  foreach my $AA (@AA) {
    unless (defined $MWs{$AA}) {
      print "'$AA' does not exist\n";
      next;
    }
    $sum += $MWs{$AA};
  }
}

print "Molecular weight of this peptide is: $sum \n";
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