use strict;
use warnings;

sub subject {
  'Subject: default subject';
}

sub to {
  'To: default_to@example.com';
}

sub from {
  'From: default_from@example.com';
}

my %config = (
  to =>      { regex => '^To:\s', sub => \&to },
  from =>    { regex => '^From:\s', sub => \&from },
  subject => { regex => '^Subject:\s', sub => \&subject },
);

my %cnf;

# Load user data
foreach (<DATA>) {
  chomp;

  foreach my $c (keys %config) {
    if (/$config{$c}{regex}/) {
      $cnf{$c} = $_;
      last;
    }
  }
}

# Look for missing values and fill in defaults
foreach (keys %config) {
  unless (exists $cnf{$_}) {
    $cnf{$_} = $config{$_}{sub}->();
  }
}

use Data::Dumper;
print Dumper \%cnf;

__DATA__
To: foo@example.com
From: bar@example.com