$ cat junk
BRCA1:breast cancer 1, early onset
TNF: tumor necrosis factor
OMG:oligodendrocyte myelin glycoprotein

$ perl -F: -lane " print $F[0]; " junk
BRCA1
TNF
OMG

$ perl -MO=Deparse -F: -lane " print $F[0]; " junk
BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    our(@F) = split(/:/, $_, 0);
    print $F[0];
}
-e syntax OK