Nice!

Here a generic - albeit slower - solution for definedness

use strict;
use warnings;
use Data::Dump qw/pp dd/;

#my $scale =42;
#our $scale =42;

sub var {
    eval $_[0];# or die $@; 
}

my $myscale = var('$scale') // 1;

pp $myscale;
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and inspecting the errorstring in $@ can help checking for existence instead.

Global symbol "$scale" requires explicit package name (did you forget to declare "my $scale"?) at (eval 1) line 1.

update

of course using eval directly is shorter

use strict;
use warnings;

#my  $scale =42;
#our $scale =42;

my $myscale = eval('$scale') // 1;

print $myscale;
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but I prefer encapsulating such magic to be able to capture side effects like polluting $@ or potential injections by tainted input.