[perl] запись в файл после чтения из него

               You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access to the file; thus '+<' is almost always
               preferred for read/write updates--the '+>' mode would clobber the file first.

you almost never want to use +> or +>>. The first clobbers your file before you can read it, and the second one is confusing because your read pointer can be anywhere, but on many systems, the writer always jumps to the end of the file.

open FH, "+< $moddir/file.lst"; 
print FH "ололо!\n"; 
while (<FH>) { 
print $_; 
}

Use of uninitialized value in subroutine entry at ./script.pl line 328, <FH> line 33.
Undefined subroutine &main:: called at ./script.pl line 328, <FH> line 33.

fenrirko ~ # cat test.pl
#!/usr/bin/perl -w
use strict;
use warnings;
use IO::File;

my $fh = new IO::File("file.lst", O_RDWR|O_APPEND);
if (defined $fh)
{
    print $fh "ололо2\n";
    $fh->seek ( 0, SEEK_SET );
    while (!$fh->eof())
    {
        print $fh->getline;
    }
    $fh->close();
}
fenrirko ~ # cat file.lst
ололо!
ололо1
fenrirko ~ # ./test.pl
ололо!
ололо1
ололо2
fenrirko ~ # cat file.lst
ололо!
ололо1
ололо2
fenrirko ~ #

#!/usr/bin/perl -w
use strict;

open FH, "+< ./file.lst";
for (my $i=0;$i<5;$i++)
{
    print FH "ололо!\n";
    while (<FH>) {
        print $_;
    }
}