As I said, the ?: makes it so the group within the () is not captured. Observe...

Without ?::

perl -E '"123" =~ /(2|3)/; say $1'
2
[download]

With ?::

perl -E '"123" =~ /(?:2|3)/; say $1'
[download]

Note how using the ?: doesn't put anything into the special numbered $1 variable. See perlretut's Non-Capturing Groupings