I'm glad my guess was along the right lines. Perhaps if I expand the code it will become clearer what is going on. The main points are: split will break a string into individual characters if given an empty pattern as it's delimiter; ord is one of those built-ins that operates by default on $_; I am using the for (synonym for foreach) as a statement modifier rather than writing an explicit loop. So, in English, concatenate $newStr with the ordinal value, formatted to three digits with leading-zero padding, of each character in the string. These characters are passed one at a time in the variable $_ to the .= statement by the for statement modifier from the list that results from the split statement.

Here's the code expanded and using temporary variables.

foreach my $char ( split m{}, $str )
{
    my $ordVal = ord $char;
    my $fixedWidthOrdVal = sprintf q{%03d}, $ordVal;
    $numStr .= $fixedWidthOrdVal;
}
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I hope that has explained the code satisfactorily but, if not, please ask.

Cheers,

JohnGG