Put it in a test script. Try it against edge cases (unusual test strings that might work when they shouldn't, or might not work when they should). For example:

use strict;
use warnings;

my @tests = (
    ".",
    "",
    "A sentence.",
    ".gitignore",
    "word.doc",
    "a.dotted.name",
    );

print /^.*(\..*)$/ ? "Matched '$1' in " : "Failed", " '$_'\n" for @tes
+ts;
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Prints:

Matched '.' in  '.'
Failed ''
Matched '.' in  'A sentence.'
Matched '.gitignore' in  '.gitignore'
Matched '.doc' in  'word.doc'
Matched '.name' in  'a.dotted.name'
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