my @words = split ' ', $line;
my @boy_idxs = grep $words[$_] eq 'boy', 0..$#words;
$words[$boy_idxs[rand(@boy_idxs)]] = 'man';
$line = join ' ', @words;

[download]

Update: This one doesn't mess up the whitespace: 

my @boy_idxs;
push @boy_idxs, $-[0] while $line =~ /\b boy \b/xg;
substr($line, $boy_idxs[rand(@boy_idxs)], length('boy')) = 'man';

[download]

@-

In reply to Re: Randomly regex substitute by ikegami
in thread Randomly regex substitute by Anonymous Monk

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