You should probably read them into a hash:

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};
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I gather you had something else in mind, such as importing the identifiers as fully fledged variables into your program, which is often (meaning almost always) a bad idea. For reference on that topic:

• perldoc perlref "Symbolic References"
• Why it's stupid to 'use a variable as a variable name': Part I
• Why it's stupid to 'use a variable as a variable name': Part II
• Why it's stupid to 'use a variable as a variable name': Part III

Symbolic references are incredibly useful under the right circumstances. And in the general course of writing a well designed Perl program those circumstances are rare.