Your example code dosn't show much, here is an example pulled from the PDL maillist. See also output from fitpoly1d

#!/usr/bin/perl
use PDL;
use PDL::MATH;
use PDL::Fit::Polynomial;

my $xdata = sequence float, 16;
my $ydata = sequence float, 16;

my $i = 0;
 while ($i < 16) {
    $xdata[$i] = $i;
    $ydata[$i] = $i * $i;
    
   print "i ", $i, " x ", $xdata[$i], " y ", $ydata[$i], "\n";
    $i++;
 }

 ($yfit, $coeffs) = fitpoly1d $xdata, $ydata, 3;

 print "Fit complete with coeffs ", $coeffs, "\n";

 print "Fitted values ", $yfit, "\n";

 $i = 0;
 while ($i < 16) {
    print "i ", $i, " yfit ", $yfit[$i], "\n";
    $i++;
 }
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