Here's one way to do it, supposing you wanted to change the number after "pref3" from 0 to 9: 
open(PREF,"+< preffile") or die;

while (<PREF>) {
    # if we see pref3 set to 0
    if (/^pref3\t0$/) {
        # you need to rewind two chars to get 
        # before the 0 and the newline , which 
        # you've already gone past
        seek(PREF,-2,1) or die;
        # print a "9" in this spot
        print PREF "9";
        # move forward one char past the newline
        # to continue reading.
        seek(PREF,1,1) or die;
    }
}
close PREF;

[download]
See open for the <+ read-write mode, and seek for the seek() bit.

Update: I should note that this only works when the replacement string is the same length as the original. See "perldoc -q 'how do I change one line'" for a complete answer.

Update2:Replaced the read() with while(<PREF>), since it's not likely that all the lines in the file are 8 chars long.

In reply to Re: How can I replace a single character (digit) in a file by kschwab
in thread How can I replace a single character (digit) in a file by sofreshsoclean

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