I would like to know why the nested maps produces the behavior?

First. Take threads and Thread::Queue out of the equation. They are innocent bystanders in the issue.

Using nested maps, this require 49MB and 11.4 seconds of cpu time to complete:

C:\test>perl -E"$c=0; map map( ++$c, 1..1e3 ), 1..1e3; say 'check mem'
+;<>"
check mem
49 MB
[download]

Whereas, this using nested for loops requires just 2 1/2 MB and 0.014 seconds of cpu:

C:\test>perl -E"$c=0; for( 1..1e3 ) { ++$c for 1..1e3 }; say 'check me
+m';<>"
check mem
2.5MB
[download]

For why,

1. map operates on lists -- so 1 .. 1e3 builds a 1000 item list on "the stack" -- and nesting them means that 1000 lists of 1000 items need to be built.
2. for will(*) process 1 .. 1e3 as an iterator, grabbing one value at a time as it needs it.

(*)for will also build a list in some circumstances, but far less frequently.

It pays to know (some of) the internal details of your language.

(As an aside, filling Thread::Queues will huge numbers of items costs big in terms of memory and runtime. Better to limit how much you push into them at one time).