Perl has a default method of reading a file one line at a time, you can then process this line and write it back out to a new file.

open FILE_HANDLE, "<", $File_Name or die "probs opening $File_Name: $!
+\n";
while (<FILE_HANDLE>) { # defaults to one line at a time
    # do something to this line that is now
    # contained in the $_ variable
}
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You can tell perl to use any string of characters as an input record seperator (the $/ variable) and then read your file one record at a time, this defaults to the end of line character for your OS so perl reads the file line at a time.

# imagine each record starts with "New Record"
local $/="New Record";
open FILE_HANDLE, "<", $File_Name or die "probs opening $File_Name: $!
+\n";
while (<FILE_HANDLE>) {
    # do something to this block now
    # contained in the $_ variable
}
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There are also various modules that tie a file on disk to a data structure without reading it all in. This may also be helpful. Have a look at Tie::File for instance.

If you give some more specific details of what you are trying to do we can give more help.

Cheers,
R.