Here is how you do it in C. I would just toss this into inline C and call it a day but it would be easy enough to convert to Perl. Note SIOCDIFADDR (to delete old address) will fail and return an einvalue on Linux as it was not implemented in the (older) kernels for IPV4. It works anyway, despite the error message. If you are on BSD or other *nix flavour it should just work.....

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>
#include <errno.h>

int main()
{
    char iface[] = "eth0";
    char ip[] = "192.168.1.3";
    IP_SetIP(iface,ip);
    return 0;
}

int IP_SetIP(const char* interface, const char * ip_address )
{
    int sock=0;
    struct sockaddr_in* addr=NULL;
    struct ifreq ifr;
    
    sock = socket( AF_INET, SOCK_DGRAM, 0 );
    if( sock == -1 ) {
        printf("Can't get a socket!");
        return (-1);
    }
    
    memset(&ifr,0,sizeof( struct ifreq ) );
    strncpy(ifr.ifr_name,interface,IFNAMSIZ);
    
    if( ioctl( sock, SIOCGIFADDR, &ifr ) < 0 ) {
        printf("Can't get IP '%s' because: '%s'\n",interface,strerror(
+errno));
    }
    else {
        if( ioctl( sock, SIOCDIFADDR, &ifr ) < 0 ) {
            printf("Can't remove '%s' because: '%s'\n",interface,strer
+ror(errno));
        }
    }
    memset( &ifr, 0, sizeof( struct ifreq ) );
    addr= (struct sockaddr_in *)&(ifr.ifr_addr);
    memset(addr, 0, sizeof( struct sockaddr_in) );
    addr->sin_family=AF_INET;
    addr->sin_addr.s_addr=inet_addr(ip_address);
    strncpy(ifr.ifr_name,interface,IFNAMSIZ);
    
    if( ioctl( sock, SIOCSIFADDR, &ifr ) != 0 ) {
        printf("Can't fix IP of '%s' with '%s' because '%s'\n",
        interface,ip_address,strerror(errno));
        close(sock);
        return (-1);
    }
    else {
        printf("IP for '%s' set to'%s'\n",interface,inet_ntoa(addr->si
+n_addr));
    }
    close(sock);
    return(0);
}
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