There is only one situation when I consciously assign the null string to a scalar:

{
    my $vec; 
    vec( $vec, 0, 1 ) = 1; 
};;
Use of uninitialized value $vec in vec at (eval 9) line 1, <STDIN> lin
+e 1.
Use of uninitialized value $vec in scalar assignment at (eval 9) line 
+1, <STDIN> line 1.

{ 
    my $vec = ''; 
    vec( $vec, 0, 1 ) = 1; 
};;
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A somewhat analogous situation arises with substr:

{ 
    my $s; 
    substr( $s, 0, 1, 'fred' ); 
};;
Use of uninitialized value $s in substr at (eval 13) line 1, <STDIN> l
+ine 5.

{
    my $s = ''; 
    substr( $s, 0, 1, 'fred' ); 
};;
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But that is less useful because it only silences the warning if there is an overlap between the range being overwritten and that that already exists:

{ 
    my $s = ''; 
    substr( $s, 1, 1, 'fred' ); 
    print $s; 
};;
substr outside of string at (eval 16) line 1, <STDIN> line 8.

{ 
    my $s = '  '; 
    substr( $s, 1, 1, 'fred' ); 
    print $s; 
};;
 fred
[download]

But beyond those few special cases, it's pretty much pointless typing.

(I also think that using q{} instead of '' is a waste of effort; and a mild form of obfuscation. PBP strikes again.)