$ cat file1
line1
line2 XXX
line3
$ cat x.c
#include <stdlib.h>
main()
{
        system("cat file1 | perl -e 'while(<>) { $_ =~ s/XXX/YYY/g; print $_;}'");
        exit(0);
}
$ gcc x.c -o x
$ ./x
line1
line2 YYY
line3
$