If you enter a non-number, it should tell you the range of numbers it thinks is acceptable¹. So, that might help to narrow down on what the problem is.

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¹ the respective code snippet is (CPAN/FirstTime.pm — update: CPAN v1.94_56):

        my $num = prompt($prompt,$default);

        @nums = split (' ', $num);
        {
            my %seen;
            @nums = grep { !$seen{$_}++ } @nums;
        }
        my $i = scalar @$items;
        unrangify(\@nums);
        if (0 == @nums) {
            # cannot allow nothing because nothing means paging!
            # return;
        } elsif (grep (/\D/ || $_ < 1 || $_ > $i, @nums)) {
            $CPAN::Frontend->mywarn("invalid items entered, try again\
+n");
            if ("@nums" =~ /\D/) {
                $CPAN::Frontend->mywarn("(we are expecting only number
+s between 1 and $i)\n");
            }
            next SELECTION;
        }
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