To retain external white space while collapsing internal white space:

use strict;
use warnings;

my $str = "  Hello\n\t\n   World  \n";
print ">$str<\n";
$str =~ s/\b\s+\b/ /g;
print ">$str<\n";
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Prints:

>  Hello
    
   World  
<
>  Hello World  
<
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