$ strace -e mmap,munmap,write ./903114.pl
...
write(2, "1", 11)                        = 1
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fe6d7315000
write(2, "2", 12)                        = 1
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fe6d5e0f000
write(2, "3", 13)                        = 1
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fe6d5d0e000
munmap(0x7fe6d5d0e000, 1052672)         = 0
munmap(0x7fe6d5e0f000, 1052672)         = 0
munmap(0x7fe6d7315000, 1052672)         = 0