Try this.

 $s ='the quick brown fox';
 $s =~ s/\b(\w+)\b/ucfirst  $1/ge;
 print $s;
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Gives

The Quick Brown Fox
[download]

Update: zigdon++ pointed out that your question showed the input as ALL UPPER CASE, which means that you would need

 $s =~ s/\b(\w+)\b/ucfirst lc $1/ge;.

Much safer.