Install Template, and you have a ready-made solution:

use Template;
Template->new({
  START_TAG => '%', END_TAG => '%',
})->process(\*DATA, { <<'END_DATA' =~ /^(\S+)\s+(.*)/gm });
from    Randal
to      Loki
subject template processing
END_DATA
__END__
Here is one way from %from% to %to% to accomplish %subject%.
[download]

-- Randal L. Schwartz, Perl hacker