you have the s/// operator in perl. this should do what you want:

while(<>) {
    s#^([^/]+).*#$1#;
    print;
}
[download]

hope this helps,

Update: even if the s/// works, split() is faster:

while(<>) {
    print((split '/')[0], "\n");
}
[download]

Update: ++Abigail-II. I need to work on my golfing skills... ;)