cprintf "There are at least %'d ways to do it!\n", 42e6;

##</code><code>##

printf "There are at least @{[commify('%d')]} ways to do it!\n", 42e6;

##</code><code>##

sub commify {
    local $_  = shift;
    my $spc = ''; s/^(\s+)// and $spc = $1; # trim and save leading space
    my $adj = 0;
    $adj++ while s/^([-+]?\d+)(\d{3})/$1,$2/;
    $spc =~ s/.{$adj}//;      # adjust space for commas added
    s/\s{0,$adj}$// if /\s$/; # adjust right padding
    return $spc . $_;
}

sub cprintf {
    (my $format = shift) =~
        s{
            \%(['+0-9.-]+)?([df]) # capture all valid %d and %f flags and modifiers
         }{
            my $p = $1;
            my $c = $2;
            $p =~ s/'// ? "\@{[commify('%$p$c')]}" : "%$p$c"
         }gex;

    my $str = sprintf $format, @_;
    print eval "\"$str\"";
}

cprintf "%+'012d\n", 1e6;
cprintf "<%-'12.6d>\n", 1e6;
cprintf "<%-+12.6'd>\n", 1e6;
cprintf "<%+12.6'd>\n", -1e6;
cprintf "<%+12.6d>\n", -1e6;
cprintf "<%+12.2'f>\n", 1234.5;
cprintf "There are at least %'d ways to do it!\n", 42e6;
__END__
+00,001,000,000
<1,000,000   >
<+1,000,000  >
<  +1,000,000>
<  -1,000,000>
<    -1000000>
<   +1,234.50>
There are at least 42,000,000 ways to do it!