sub foo{
    my $var = 'xyz' x 1_000_000;
}

sub bar{
    my $var2 = 'xyz' x 1_000_000;
}

warn "ONE\n";
foo();
warn "TWO\n";
foo();
warn "THREE\n";
bar();
warn "FOUR\n";
[download]

Running this through strace gives this output:

[...]
write(2, "ONE\n", 4)                    = 4
mmap2(NULL, 3002368, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 
+-1, 0) = 0xb7956000
mmap2(NULL, 3002368, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 
+-1, 0) = 0xb7679000
write(2, "TWO\n", 4)                    = 4
write(2, "THREE\n", 6)                  = 6
mmap2(NULL, 3002368, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 
+-1, 0) = 0xb739c000
mmap2(NULL, 3002368, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 
+-1, 0) = 0xb70bf000
write(2, "FOUR\n", 5)                   = 5
[download]

This shows that perl does not allocate memory between "TWO" and "THREE" (the variable in sub foo gets reused), but it does allocate new memory before running bar, even though it could reuse $var's memory for $var2.

Including system('ps', '-Orss', '-p', $$) and die "$! $?"; in various places leads to the same conclusion without strace.