use strict;
use warnings;

my ($text, %var);

$var{VAR1} = 'hawg';
$var{VAR2} = 'mansion';
$var{VAR3} = 'flagpole';

read(DATA, $text, 8192);
$text =~ s/<\?(\w+?)\?>/$var{$1}/g;
print $text;

__DATA__
Once upon a time there was a <?VAR1?> that lived in a
<?VAR2?> on top of a <?VAR3?>.