The most readable solution (at least for me) uses just two regexes, the first one shrinks the backslashes and replaces them with slahses, the second one removes the trailing slashes:

my $script_dir = 'C:\\temp\\\\';
print "1. $script_dir\n";

$script_dir =~ s{\\+}{/}g;
print "2. $script_dir\n";

$script_dir =~ s{/$}{}g;
print "2. $script_dir\n";
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You can use transliteration with shrinking instead of substitution in the first step:

$script_dir =~ tr{\\}{/}s;
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It's also possible to do everything in one step, but we're entering a golf course here:

$script_dir =~ s{\\+(?=(.)?)}{"/"x defined $1}ge;
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Update: Talking about golf, this is shorter:

$_ = s/\\+$//r =~ s{\\+}{/}gr for $script_dir;
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