If you name your constants file something with a .PL on the end, EU::MM will automatically run it for you. You could even generate your .pm directly this way.

# my-constants.PL

open my $pm, ">", "my-constants.pm" or die "Can't open my-constants.pm
+: $!";
open my $h, ">", "my-constants.h" or die "Can't open my-constants.h: $
+!";


print { $pm } "# Generated content follows. Edit $0 instead.\n"
  or die "Can't write to my-constants.pm: $!";
print { $h } "/* Generated content follows. Edit $0 instead. */\n"
  or die "Can't write to my-constants.h: $!";

while ( my $const = <DATA> ) {
    chomp $const;
    my ( $name, $val ) = split ' ', $const, 2;

    print { $pm } "use constant '$name' => $const;\n"
      or die "Can't write to my-constants.pm: $!";
    print { $h } "#define $name $const\n"
      or die "Can't write to my-constants.h: $!";
}

print { $pm } "1; # End of generated my-constants.pm\n"
  or die "Can't write to my-constants.pm: $!";
print { $h } "/* End of generated my-constants.h */\n"
  or die "Can't write to my-constants.h: $!";

close $pm
  or die "Can't flush my-constants.pm: $!";
close $h
  or die "Can't flush my-constants.h: $!";

__DATA__
FOO 1
BAR 2
BAZ 3
[download]

Produces my-constants.pm

# Generated content follows. Edit my-constants.PL instead.
use constant 'FOO' => 1;
use constant 'BAR' => 2;
use constant 'BAZ' => 3';
1; # End of generated my-constants.pm
[download]

And my-constants.h

/* Generated content follows. Edit my-constants.PL instead. */
#define FOO 1
#define BAR 2
#define BAZ 3
/* End of generated my-constants.h */
[download]