use POSIX qw( LOCK_EX SEEK_SET );

# Open a shared file for reading and writing
open my $fh, '<+', $filename
   or die "Can't open $filename for reading: $!";
flock $fh, LOCK_EX | LOCK_NB
    or die "Can't get an exclusive lock on $filename: $!";

# Read one line.
my $contents = <$fh>;

# Empty the file.
seek $fh, 0, SEEK_SET
    or die "Can't seek to the start of $filename: $!";
truncate $fh, 0
    or die "Can't truncate $filename: $!";

# Put something new in there.
print $fh "...."
    or die "Can't write to $filename: $!";
close $fh
    or die "Can't write to $filename: $!";