TIMTOWTDI but using the core Time::Piece module makes it pretty trivial:

#!/usr/bin/env perl 
use strict;
use warnings;
use Time::Piece;

my @lines = (
qq{Misc data that is unimportant:false,true,false,"retrieved_on":14335
+07459,more misc data,nul;,true,"created_utc":"1433122764",\n},
qq{"retrieved_on":1433507472,misc data,"created_utc":"1433122764",true
+,false,more misc data\n}
);

foreach (@lines) {
    $_ =~ s/([\d]{10})/(localtime $1)->datetime/eg;
    print;
}
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