open (INFILE, "input.txt")
  or die "Can't open input.txt: $!\n";
open (OUTFILE, ">output.txt")
  or die "Can't open output.txt: $!\n";

while (<INFILE>) {
  s/foo/bar/g;
  print OUTFILE;
}
[download]

I removed the /m and /s options from your s/// operator as they have no effect on data that doesn't contain newlines.

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg

open (INFILE, "input.txt") or die "cannot open input: $!";
open (OUTFILE, ">output.txt") or die "cannot open output: $!";

while (<INFILE>) {   # note the < and > brackets

   s/foo/bar/sg;     # no =~ needed for $_
                     # no m needed

   print OUTFILE;    # no "$_\n" needed: $_ is automatic
                     # and the \n is still in $_ (you
                     # don't cho(m)p it...
      
}

close INFILE;
close OUTFILE;
[download]

If you need to replace a multi-line string, you might try setting $/ to undef (slurp the whole file) or "" (read in paragraphs), and maybe adding a /s modifier to the replacement regex.

I'd recommend reading:

perldoc -f open
perldoc perlvar (look for the $/ entry)
perldoc perlre (for info about regular expressions)
[download]

Update: added or die, and close parts, trying not to set a bad example :-)

-- 
Joost       downtime n. The period during which a system
            is error-free and immune from user input.
[download]

I understood that your need is to read a file, line by line, and if the line is not blank, write the line to the output file. Did I understand that correctly?

My code snippet, as you have quoted above, will:

• Read one line from the input file
• Remove a potential newline from the end of the line
• Look if the line is now empty (zero length)
• If empty, read next line
• If not empty, write the line and a newline to the output file and then read next line from the input file
• Repeat until no more lines in input file

while(<IN_FILE>)
   s/from/to/g;
   print OUT_FILE;
   }
[download]

Your suggestion did work; thank you! Now I'm trying to apply the same principle to finding and replacing text.

open (INFILE, "input.txt"); open (OUTFILE, ">output.txt"); while (INFILE) { $_ =~ s/foo/bar/gms; print OUTFILE "$_\n"; }

/m is multi line: not useful when dealing with a single line at a time. /s makes . match newline: not useful if you don't use ..

One liner:

perl -i.backup -pe's/foo/bar/g' filename
[download]

Same thing, inside a larger script that you don't want to have using -i all the time:

{
    local @ARGV = ('filename');
    local $^I = '.backup';
    local $_;
    while (<>) {
        s/y/yah/g;
        print;
    }
}
[download]

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

I fear this is an especially dumb question, but here goes...

I've seen a lot of people recommending the "perl -i" solution for regexes, like your

perl -i.backup -pe 's/foo/bar/g' filename
[download]

Can that be used for multiple regexes in a file? Like, if you wanted to do s/foo/bar/g and s/y/yah/g on the same file?

I rescind the question. Yes, it's dumb, and yes, I figured it out by doing a search for "perl -i". Sorry.

Can that be used for multiple regexes in a file? Like, if you wanted to do s/foo/bar/g and s/y/yah/g on the same file?

Yes. See what happens if you use just perl -pe'undef':

LINE: while (defined($_ = <ARGV>)) {
    undef;
}
continue {
    die "-p destination: $!\n" unless print $_;
}
[download]

So you can just add whatever should be inside the block:

perl -i -pe's/foo/bar/g; s/y/yah/g;' filename
[download]

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.