I tried to combine the second and fourth lines of the snippet,

You know that the start dir is going to be followed by a '\', so you could do $plsname =~ s/^\Q$startdir\E\\//i; and the fourth line becomes redundant.

You could combine the 5th line at the same time: $plsname =~ s[^\Q$startdir\E\\(.+)$][$1.pls]i

The /g on the third line makes that pretty much impossible to combine with the rest; and single character substitutions are better done with tr/// giving:

my $plsname = $curdir;

$plsname =~ s[^\Q$startdir\E\\(.+)$][$1.pls]i;

$plsname =~ tr[\\][_]; 

print $plsname;;
Schubert_Lieder_Terfel.pls
[download]

Is that better? Your call.