lvalue vs rvalue. When an undefined value is dereferenced to be used as an lvalue, it gets autovivified.

>perl -Mstrict -wle"my $r; my @x; @x=@$r; print $r"
Can't use an undefined value as an ARRAY reference at -e line 1.

>perl -Mstrict -wle"my $r; my @x; @$r=@x; print $r"
ARRAY(0x225280)
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map appears to expect an lvalue, possibly because it aliases.

>perl -Mstrict -wle"my $r; my @x = map{ $_ } @{ $r }; print $r"
ARRAY(0x225258)
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More examples where aliasing is involved:

>perl -Mstrict -wle"my $r; my @x; for(@$r){} print $r"
ARRAY(0x225280)

>perl -Mstrict -wle"my $r; my @x; sub{}->(@$r); print $r"
ARRAY(0x225280)
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Update: Oops, fixed mention of rvalue where I meant lvalue. Re-ordered post for better readability. Added extra examples.