Could also be coded...this does the same thing:
...
my @clk_new = map{my $line =join ("_", @$_); $line}@clk_output;

But isn't it also true that the statement
    my @clk_new = map{my $line =join ("_", @$_)}@clk_output;
would do the same thing? In which case, there seems to be no point to assigning to a lexical within the map block, so we're back to
    my @clk_new = map{ join('_', @$_) }@clk_output;
or
    my @clk_new = map join('_', @$_), @clk_output;

Prior to the introduction of the  /r modifier for  s/// substitution in Perl version 5.14 (see Regexp Quote Like Operators in perlop), there was sometimes a need for a map statement like
    map { (my $r = $_) =~ s{ ... }{foo}xms;  $r; }
to avoid changing the aliased referent of  $_ in a data flow, but it doesn't seem useful here. (I can't tell you how many times I've had to remember to do this when a bunch of unexpected '1' characters suddenly showed up in my output!)

A similar situation might arise if one wanted to stick a debug print-point
    map { print "... $_ ...";  $_; }
into the middle of a data flow.