Further to some of BrowserUk's comments ++above:

• A  \ (backslash) within a  \Q...\E sequence: if the backslash is included in the interpolated variable, things are simpler:

  c:\@Work\Perl\monks>perl -wMstrict -le
  "my $curdir   = 'Y:\Music\Schubert\Lieder\Terfel';
   my $startdir = 'Y:\mUsIc\\';
   ;;
   my $plsname = $curdir;
   $plsname =~ s/^\Q$startdir\E//i;
   $plsname =~ s/\\/_/g;
   $plsname .= '.pls';
   print qq{'$plsname'};
  "
  'Schubert_Lieder_Terfel.pls'
  [download]

  (Note that the final  \ (backslash) in the  $startdir assignment has to be escaped because it's just before the closing single-quote delimiter.)
• Some examples of  my ($plsname) = $curdir =~ regex; (still not applicable):

  c:\@Work\Perl\monks>perl -wMstrict -le
  "my $s = 'bar foooo bozzle';
   ;;
   my ($capture) = $s =~ m{ (fo+) }xms;
   print qq{'$capture'};
   ;;
   my ($found_it) = $s =~ m{ (?<= \s) b .* e }xmsg;
   print qq{'$found_it'};
   ;;
   my $regex = qr{ \A (b \w+) }xms;
   my ($it_was_there) = $s =~ $regex;
   print qq{'$it_was_there'};
  "
  'foooo'
  'bozzle'
  'bar'
  [download]

  (The second example without explicit capture groups.) (Update: Changed code above to add  $regex example.)
• Also note that BrowserUk is very experienced at dealing with Big Data. If you're operating on a single string of a few dozen/hundred/thousand characters, the difference between  s///g and  tr/// will not be detectable | noticeable in practice.
• As to docs, all the usual suspects: perlre, perlretut, perlrequick, perlrecharclass, perluniprops; in perlop: Quote and Quote-like Operators, Regexp Quote-Like Operators, Quote-Like Operators. (Update: Also see Pattern Matching, Regular Expressions, and Parsing in our very own Tutorials.)

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