perl -wle 'print ":" . XXX'

##</code><code>##

perl -wle '$_ = "345678"; print ":" . /./g'

##</code><code>##

perl -wle '$_ = "345678"; print ":" . ( () = /./g )'
# output: ':6' (6 - number of matches; special behaviour)

##</code><code>##

perl -wle '$_ = "345678"; print ":" . @{[ /./g ]}'
# output: ':6' (6 - number of matches => number of array elements)

##</code><code>##

perl -wle '$_ = "345678"; print ":" . @_{ /./g }'
#Use of uninitialized value in concatenation (.) or string at -e line 1.
#:

##</code><code>##

perl -wle '$_ = "345678"; print ":" . ( /./g )[ 1 ]'
# output: ':4' (second match for e.g. index 1)

##</code><code>##

perl -wle '$_ = "3" x 5e5; print ":" . method( XXX )'

##</code><code>##

perl -wle '$_ = "345678"; print ":" . ( /./g )[ [] ]'
#Use of uninitialized value in concatenation (.) or string at -e line 1.
#:

##</code><code>##

perl -wle '$_ = "345678"; print ":" . ( /./g )[ undef ]'
#Use of uninitialized value in list slice at -e line 1.
#:3

##</code><code>##

perl -wle '$_ = "345678"; print ":" . ( /./g )[()]'
#Argument ":" isn't numeric in list slice at -e line 1.
#:3

##</code><code>##

perl -wle '$_ = "345678"; print ":" . ( /./g )[@_]'
#Argument ":" isn't numeric in list slice at -e line 1.
#:3

##</code><code>##

perl -wle '$_ = "345678"; print "2" . ( /./g )[()]'
#25

##</code><code>##

perl -wle '$_ = "345678"; print "2" . ( /.(?{ print $& })/g )[()]'
3
4
5
6
7
8
25