You are missing a bracket:

-0.137 + (0.00479 / (W/0.9)) + (2.5593 * (W/0.9 ) ,0.045,0 

-0.137 + (0.00479 / (W/0.9)) + (2.5593 * (W/0.9 ) <---- HERE ,0.045,0
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I still dont understand how this works - but it does!

Update: Including testing code that ( I am not sure how ) works!

use strict           ;
use warnings         ;

sub EvalPolynominal{
    #first parameter gives the polynominal string
    #second parameter gives the min-des width
    #third parameter gives the short-line length
    
    my $polynominal=shift;
    my $curWidth=shift;
    my $curLength=shift;
    
    $polynominal =~ s/W/$curWidth/g ;#replace W with current value of 
++min width
    $polynominal =~ s/L/$curLength/g ;#replace L with current value of
++ min Length

    my $result ;
    eval( $result = $polynominal ) ;
    # print "$result\n"; -- PRINTS THE STRING 

    return eval ($polynominal); # Returns '0.086765' - WHY?
}


my $one = EvalPolynominal( '-0.137 + (0.00479 / (W/0.9)) + (2.5593 * (
+W/0.9 ))' ,0.045,0 );

print "$one\n";
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