You'd probably use sysopen with seek and truncate, but that's probably still not what you really want to do. Read the entries in perlfaq5 on file locking and flock if you want to do this correctly.

Here's an example.

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: $!";
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