$ 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