use Modern::Perl;

sub _lc {
    my $key = shift;
    my $success = $key =~ s/^-?([^-_]*)[-_]?([^-_]*)$/$2?lc"$1_$2":lc$1/e;
    return $key if $success;
}

print _lc($_) while (<DATA>);

__DATA__
content-length
content_length
-content-length
-content_length
Content-Length
Content_Length
-Content-Length
-Content_Length
changed
Changed
-changed
-Changed
Too-many_dashes-Or-underscores