I can reproduce what you describe. Firstly, two text files which only contain the string 'pl'.

$ cat junk
pl
[download]

$ cat junk2
pl
[download]

One matches your regex, the other doesn't:

$ perl -nE 'say +(/p.{2,6}/) ? "match" : "no match"' junk
match
[download]

$ perl -nE 'say +(/p.{2,6}/) ? "match" : "no match"' junk2
no match
[download]

A closer look inside the files explains why.

$ cat -vet junk
pl^M$
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$ cat -vet junk2
pl$
[download]

[In case you're unfamiliar with the command, cat -vet, displays carriage returns as "^M" and newlines as "$".]