use 5.010 'uses //';
Perl v5.10.0 required (uses //) -- this is only v5.12.1, stopped.
# No, this doesn't actually work.

##</code><code>##

BEGIN { die "// not implemented in Perl $]\n" if $] < 5.010 }

##</code><code>##

BEGIN { die "// not implemented in Perl $]\n" if ! eval "undef() // 1" }

##</code><code>##

perl5.8 -e'my $x= undef() // 1;'
Search pattern not terminated ...

##</code><code>##

use v5.14 "Your Perl is old. Please don't waste my time. Go away until you upgrade.";