G'day taint,

Here's another way to do it:

$ perl -Mstrict -Mwarnings -E '
    my @files = qw{abc1.html abc12.html abc123.html abc1234.html};
    say for map {
        s/^(abc)(\d{1,3})(\.html)$/$1 . "0" x (4 - length $2) . $2 . $
+3/e; $_
    } @files;
'
abc0001.html
abc0012.html
abc0123.html
abc1234.html
[download]

Update: "... the files begin with the same 3 alpha characters ..." so \w{3} is far too generic: s/\w{3}/abc/