This might suffice. It relies on the continuation lines starting (after the leading whitespace) with at least 3 non-space, non-':' characters.

#! perl -slw
use strict;

my $def =<<'EOD';
bean
     n 1: any of various edible seeds of plants of the family
          Leguminosae [syn: {edible bean}]
     2: any of various seeds or fruits suggestive of beans
     3: any of various leguminous plants grown for their edible
        seeds and pods [syn: {bean plant}]
     4: informal terms for a human head [syn: {attic}, {bonce}, {noodl
+e},
         {noggin}, {dome}]
     v : hit on the head, esp. with a pitched baseball
EOD

$def =~ s[\n\s+(?=[^\s:]{3})][ ]smg or warn 'no match';

print $def;

__END__
c:\test>junk
bean
     n 1: any of various edible seeds of plants of the family Legumino
+sae [syn: {edible bean}]
     2: any of various seeds or fruits suggestive of beans
     3: any of various leguminous plants grown for their edible seeds 
+and pods [syn: {bean plant}]
     4: informal terms for a human head [syn: {attic}, {bonce}, {noodl
+e}, {noggin}, {dome}]
     v : hit on the head, esp. with a pitched baseball
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