c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.014;
 ;;
 my $curdir   = 'Y:\Music\Schubert\Lieder\Terfel';
 my $startdir = 'Y:\mUsIc\\';
 ;;
 my $plsname =
   $curdir =~ s{ \A (?i) \Q$startdir\E | \\ }{ $-[0] ? '_' : '' }xmsreg . '.pls';
 print qq{'$plsname'};
"
'Schubert_Lieder_Terfel.pls'

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 my $curdir   = 'Y:\Music\Schubert\Lieder\Terfel';
 my $startdir = 'Y:\mUsIc\\';
 ;;
 my %replace = ('1', '',  '-1', '_',  '0', '.pls');
 (my $plsname = $curdir) =~
   s{ \A (?i) \Q$startdir\E | \\ | \z }
    {$replace{ $-[0] == 0 || $-[0] <=> $+[0] }}xmsg;
 print qq{'$plsname'};
"
perl version 5.008009
'Schubert_Lieder_Terfel.pls'

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 my $curdir   = 'Y:\Music\Schubert\Lieder\Terfel';
 my $startdir = 'Y:\mUsIc\\';
 ;;
 my %replace = ('1', '',  '2', '_',  '3', '.pls');
 (my $plsname = $curdir) =~
   s{ (\A (?i) \Q$startdir\E) | (\\) | (\z) }
    {$replace{ $#- }}xmsg;
 print qq{'$plsname'};
"
perl version 5.008009
'Schubert_Lieder_Terfel.pls'