my %vals;
open my $infile, '<', 'data.txt' or die $!;
while( <$infile> ) {
    chomp;
    my( $identifier, $value ) = split /\s*=\s*/;
    $vals{$identifier} = $value;
}
close $infile;

my $calculation = $vals{x1} * $vals{y1};
$calculation = ( $vals{x1} * $vals{y1} ) / $vals{x2};