# set up a hash containing the values you want
my %SWAPS = ();
$SWAPS{printer} = $printer;
$SWAPS{quota} = $quota;
if($pagestate eq "duplex"){
  $SWAPS{pagestate} = 'double sided';
}elsif ($pagestate eq "simplex"){
  $SWAPS{pagestate} = 'single sided';
}
if( 1 ){
  $SWAPS{multiplier} = 'some_multiplier';
}

# swap out things like __printer__ with $SWAPS{printer}
$msg =~ s/__(\w+?)__/$SWAPS{$1}/g;