pvighneshmufc:

He meant that the following lines are inside of a loop, like this:

# read file1 into a %hash
... code to do that here ...

# inside a loop
while (my $line = <$file2>) {
    # read file2 line by line
    ... this was done in the loop condition above ...

    # split the $line to @fields at |

    # if the first $fields[0] exists in the %hash,
        # print the whole $line to file 3
}
[download]

This is a relatively common question, so LanX gave you the outline of a good solution to the problem.

A frequent mistake is to try to read *both* files inside the loop, giving one of two bad outcomes:

• Either the first file is completely read in the first pass of the loop, so the code can only find a single match if it happens to be the first line in the second file, or
• the code re-opens the first file each time, and therefore can find all the matches, but runs extremely slowly⁽¹⁾ because it reads the first file completely for each line in the second file.

(1) Extremely slowly in the relative sense--for small files you may not notice it. But if your files get large enough, you'll wonder why such a fast computer is so freakin' slow.

...roboticus

When your only tool is a hammer, all problems look like your thumb.