What to do with "Argument is not numeric" in Gtk3 Perl?

Question

Perl

Hello!
The bottom line is this: I'm playing with Gtk3 Perl, installed, wrote the following program:
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But at startup I get this error:
Argument "" isn't numeric in numeric gt (>) at Astronomy.pm line 10 (# 1)
(W numeric) The indicated string was fed as an argument to an operator
that expected a numeric value instead. If you're fortunate the message
will identify which operator was so unfortunate.
Argument "" isn't numeric in subtraction (-) at Astronomy.pm line 14 (#1)
Argument "" isn't numeric in division (/) at Astronomy.pm line 17 (#1)
It doesn't even google!
And when you click on the "Calculate" button, this error appears:
*** Not a CODE reference at /usr/share/perl5/Gtk3.pm line 318.
*** ignoring at /usr/share/perl5/Gtk3.pm line 318.
The program runs, but I can't compute what I need maybe. There is some number in the Label, but when you click on the button, it does not change.
Tried $text_jd->set_label("Julian day is: $jd"); replace with print("$jd"); - the same error came out, about "unhandled exception ..."
Thanks in advance to all who responded!