As ikegami has mentioned, you have to remember what "map" returns - as the docs say, it's an evaluation of each operation within the block as a list (in my experience, the "s///" inside a "map" is one of the most common mistakes that new users of "map" make.)

Incidentally, since you're already using a regex inside your block, you don't really need that "split":

#!/usr/bin/perl -w
use strict;

my @teststr = (
"32977186    4 -rw-r--r--   1 owner   mygrp   65 Aug  4 13:16 /long/pa
+th/to/my/file/file1.txt",
"32977186    4 -rw-r--r--   1 owner   mygrp   65 Aug  4 13:16 /long/pa
+th/to/my/file/file2.txt",
"32977186    4 -rw-r--r--   1 owner   mygrp   65 Aug  4 13:16 /long/pa
+th/to/my/file/file3.txt",
"32977186    4 -rw-r--r--   1 owner   mygrp   65 Aug  4 13:16 /long/pa
+th/to/my/file/file4.txt",
);

my %testhash = map { m{(([^/]+)\.[^/]+)$}; $2, $1 } @teststr;

print "$_ => $testhash{$_}\n" for sort keys %testhash;
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Output:

file1 => file1.txt
file2 => file2.txt
file3 => file3.txt
file4 => file4.txt
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