Just two suggestions:

1. Don't use loose .pl files. Make proper modules, with their own namespaces. perlmod is your friend.
2. Don't make assignment to variables inside your modules. Return the value of the calculation from a function, and use that.

What you have will be something like:

use strict;
package Module1;
use Exporter 'import';
our @EXPORT_OK = qw(compute_value);

sub compute_value {
  # do the computation
}

1; # this is required, see perldoc perlmod
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in the file "Module1.pm" and something like:

use strict;
use Module1;

my $variable1 = Module1::compute_value();
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or:

use strict;
use Module1 qw(compute_value);

my $variable1 = compute_value();
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in your main script.