The error is pretty self-describing. $var contains undef. If you don't want undef to be passed to the invocation of $funct, you can find out where it is passed by using:

use Carp 'croak';

my $funct= sub {
    my $var= shift;
    croak '$var was passed an uninitialized value'
        if not defined $var;
    ...
};
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Note that the prototype on the subroutine won't help anything because a prototype only influences how Perl parses source code and Perl does not know where the value of $funct is used to call a function. I would leave the prototype off.