For better readability of the code, how can I rename the $_ variable.

Well, one way might be with English:

my @x = map {$_*2} 1..3;
# --becomes-->
use English;
my @y = map { $ARG * 2 } 1..3;
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But I think the point of map and $_ is to be relatively short, so if you want to improve readability by being a bit more verbose, the general pattern for that is:

my @x = map {sprintf("%02x",$_)} 45..48;
# --becomes-->
my @y;
for (45..48) {
    push @y, sprintf("%02x",$_);
}
# --becomes-->
my @z;
for my $val ( 45 .. 48 ) {
    push @z, sprintf "%02x", $val;
}
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I would rather know step by step exactly what the code is doing ... I do not see the above code having a list passed to it.

Here are two debugging techniques to help with that: First, Data::Dump (like Data::Dumper but with a bit nicer output), for example:

use Data::Dump;
dd( unpack "CCCCCC", "\xab\xcd\x00\x1d\x94\x56" );
__END__
(171, 205, 0, 29, 148, 86)
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So you see that unpack is returning a list. Second, if you're unsure what context a function is being called in, like localtime(time) in the following example, then to test it you can replace the function call with the whatcontext function I show below, which makes use of wantarray.

my  $q  = localtime(time);
my ($r) = localtime(time);

sub whatcontext {
    print wantarray ? "list" :
        defined wantarray ? "scalar" : "void", "\n";
    return;
}

my  $s  = whatcontext;
my ($t) = whatcontext;
whatcontext;

__END__

scalar
list
void
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