For example, if you are in the directory /home/you and you want to open file.txt in the directory /home/me, you could open the file in 3 different ways:

open(FILE, "/home/me/file.txt") or die "$!"; #absolute path

open(FILE, "../me/file.txt")    or die "$!"; #relative path

chdir("../me") or die "$!";
open(FILE, "file.txt")          or die "$!"; #current dir
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# open a file for reading in the current directory
open(FILEHANDLE,"< some_file");

# open a file for reading using the full path (always a good idea)
open(FILEHANDLE,"< /home/my_user/some_file");

# open a file for writing
open(FILEHANDLE,"> /home/my_user/some_file");

# append to the end of a file
open(FILEHANDLE,">> /home/my_user/some_file");

# and it's always a good idea to check for failures ($! contains the e
+rror message)
open(FILEHANDLE,"< /home/my_user/some_file") || die "Can't read file: 
+$!";

# be polite and close the file when you're done :)
close(FILEHANDLE);
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# by the way

# read a line from a file
$line = <FILEHANDLE>;

# write something to a file
print FILEHANDLE "some line of text\n";
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/\/\averick

The usage of paths, and the paths that you use, differ slightly from OS to OS. MacPerl, for example, has rather interesting logic to determine whether the argument you've provided to open is a relative path or absolute path, etc. If you're just on Unix, though, it should be fairly straightforward.

$input_file="/home/foo/foo_file.txt";
open (INFILE, $input_file) || die "Could not open $input_file\n";

while(<INFILE>){
    print $_;
}

close INFILE;
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