use Fcntl qw(:flock);
use Symbol;

my $fh = gensym;
open $fh, "/tmp/foo" or die $!;

# start critical section
flock $fh, LOCK_SH;  # shared lock, appropriate for reading
seek $fh, 0, 0;
my @lines = <$fh>;
for (@lines) {
    print if /foo/;
}
close $fh; # close unlocks the file
# end critical section