You could use join ',',@list, but that wouldn't get you the "and" at the end...

Untested code:

@thingsicouldknow = ('this','that','the other','the last thing');
@iknowthat = (0,1,3); #this, that, and last thing

# now print the things I know...
@thingsiknow = @thingsicouldknow[@iknowthat];
print "I know about "
print join(', ',@thingsiknow[0..@thingsiknow-2]);
print ", and", @thingsiknow[-1] if @thingsiknow > 1;
print ".\n".
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It's probably not the best, but it might work. Update:Fixed some off-by-one errors caused by using $#foo

This version should not print the "and" if you only know one thing.