#!/usr/bin/perl
use strict; use warnings;

open my FILE, '<', $ARGV[0] or die "Can't read $ARGV[0]: $!";

my %header;
while (<FILE>) {
   chomp; # trim trailing newline
   my ($k, $v) = split(': ', $_, 2);
   $header{$k} = $v;
}

# now $header{To} eq 'san<sanjay@36.212.176.07>' &etc.