if ( /^$fname/ eq "s" )
{
# how do I do the substitution??
printf "hello\n";
}
[download]

$fname =~ s/^s/_S/;
[download]

Thank you!

Your attempted comparison is way off. You're trying to use the filename you want to match against as a regular expression, the result of that comparison you're for some reason trying to compare with the string "s". You need to use the =~ operator to bind a match /^s/ to the variable holding the filename ($fname =~ /^s/; see perlretut). As for the substitution, you should look at the docs for the substitution operator. And unrelated, but you probably want print not printf.

And you might look for some better introductory material (see http://learn.perl.org/ for example).

Update: Actually Regexp Quote-Like Operators in perlop is probably a better source for info on s///.

There are two major ways to ask the question "Does this string start with S?" in Perl. One method is to use a regular expression. The other is to use substr.

#!/usr/bin/perl 
use strict;      # glad to see you using strict.  Good job!
use warnings;    # lexical warnings are better the the -w switch, 
                 # becuase you can turn them off where appropriate.
                 # Good job using warnings, too.
use diagnostics; # diagnostics give verbose explanations of error mess
+ages.
                 # It can be very handy when you are just starting out
+.


# You had: open FNAMES,"</var/tmp/flist";
# I made several changes here:
# *  I switched to the 3 argument open.  It's avoids some potential 
#    security issues, and is generally considered to be the best 
#    way to go. 
# *  I am also using a lexical filehandle instead of a global handle.
#    This is generally considered to be the best way to go as well, 
#    for the same reasons that we prefer lexical variables with 
#    limited scope to global variables.
# *  Finally, I am checking the results of the open() call.  If the 
#    call fails I end the program and display an error message.  

open( my $filenames, '<', '/var/tmp/flist' )
    or die "Unable to open file list - $!\n";

while ( defined( my $fname = <$filenames> ) ) {
    chomp ($fname); # Added a semicolon


    # Regex based approaches:
    
    # Does the string start with an "s"?
    if ( $fname =~ /^s/ ) {
        $fname =~ s/^s/_S/; # Do the substitution

        # Do special stuff 
    }


    # Did we replace a starting 's' with '_S'?
    if ( $fname =~ s/^s/_S/ ) {
        # Do special stuff if we did a substitution.
    }

    # Using substr()
    #
    # Does the string start with an "s"?
    if ( 's' == substr($fname, 0, 1) ) {
        substr($fname, 0, 1) = '_S'; # Do the substitution
        # or
        substr($fname, 0, 1, '_S');  # Do the substitution

        # Do special stuff 
    }

    # Does the string start with an "s"?
    if ( 's' == substr($fname, 0, 1) ) {
        substr($fname, 0, 1) = '_S'; # Do the substitution
        # or
        substr($fname, 0, 1, '_S');  # Do the substitution

        # Do special stuff 
    }
}
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For this situation, I'd just do the substitution with a regex in the condtional test: if ($fname =~ s/^s/_S/) .... In fact, in most cases a regex is the way to go for looking into strings and finding stuff inside them. But sometimes substr is the way to go.

You've made a good start. Keep working, and you will see good results.

http://www.cpan.org/authors/id/F/FO/FORMAN/ren-regexp-1.5

I also found (but have never used) a similar CPAN module: File::Rename

There is also: How do I do a bulk rename?