The return value of s/// is not the changed string but the number of successful substitutions.

In the case of your s/^-// the return value will always be 0 or 1.
But in the case of:   s/something/something else/g   it could easily be a higher number.

my $string = 'abacadaeaf';
my $result = $string =~ s/a/x/g;
print $result;            # prints 5
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Update: Note that wog is correct in saying that a successful match will return a TRUE value. But it can be something other than 1 if /g is specified.