Not sure why you're using substr.  Let me suggest that you keep the index and the basename separate.

For example:

my $index = 0;
for (my $i = 0; $i < 100; $i++) {
    $index++;
    my $fullname = sprintf "base_%04d", $index;
    printf "fullname = '%s'\n", $fullname;
}
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will print 100 different names, starting with "name_0001" and ending with "name_0100".  The %04d achieves this result; the "0" left-pads the integer value with zeroes, and the "4" gives you 4 decimal places.

Read up on sprintf to get more details:  perldoc -f sprintf.

If you absolutely must get the last 4 digits of a string, you can use "substr" like this:

$last_4_chars = substr($my_string, -4);
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This has the effect of taking the "rest" of the string, starting at the 4th character from the end (since the offset -4 is negative) rather than from the beginning.