chdir("e:\\perl\\comics\\");
foreach my $fname (glob("*.png")) {
    my $append = "0" x (7 - length($fname));
    rename($fname, $append.$fname) or die "Cannot rename $fname";
}

##</code><code>##

perl -e 'rename($_,("0"x(7-length($_))).$_) for glob("*.png")'

##</code><code>##

rename($_,substr("000$_",-7))