my $N = 1;  # Number of replacements.
my $a = "boy boy girl boy girl boy girl girl";
my @b = split ' ', $a;
my $c = grep {$_ eq "boy"} @b;
my @d = map {$_ eq "boy" && rand($c--) < $N ? do {$N--; "man"} : $_} @
+b; 
say "@d";
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