I would not set a global variable from a subroutine (This would be very un-OOP like, not that this is OOP anyway). And you don't need to quote the file when opening it:
my @contents = &open_file(myfile.txt);
sub open_file {
open(FILE, $_[0]) or die "could not open FILE $_[0]: $!";
my @contents = <FILE>;
close(FILE);
@contents;
}