$ strace -otrace_ok  perl -MDBD::Pg -e1 
$ ulimit -Sv 16000    # this simulates your non-working environment
$ strace -otrace_err perl -MDBD::Pg -e1 
Can't load '/usr/local/perl/5.8.8/lib/site_perl/5.8.8/x86_64-linux/auto/DBD/Pg/Pg.so' for module DBD::Pg: libpq.so.3: failed to map segment from shared object: Cannot allocate memory at /usr/local/perl/5.8.8/lib/5.8.8/x86_64-linux/DynaLoader.pm line 230.
 at -e line 0
Compilation failed in require.
BEGIN failed--compilation aborted.

##</code><code>##

----- working case -----
(...)
open("/usr/local/perl/5.8.8/lib/site_perl/5.8.8/x86_64-linux/auto/DBD/Pg/Pg.so", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300_\0\0"..., 832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=93669, ...}) = 0
mmap(NULL, 1127840, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4, 0) = 0x2b3b10e2a000
madvise(0x2b3b10e2a000, 1127840, MADV_SEQUENTIAL|0x1) = 0
mprotect(0x2b3b10e3b000, 1048576, PROT_NONE) = 0
mmap(0x2b3b10f3b000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 4, 0x11000) = 0x2b3b10f3b000
close(4)                                = 0
open("/usr/local/postgresql-7.4.5/lib64/tls/x86_64/libpq.so.3", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/tls/libpq.so.3", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/x86_64/libpq.so.3", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/libpq.so.3", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0o\0\0\0"..., 832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=131049, ...}) = 0
mmap(NULL, 1158680, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4, 0) = 0x2b3b10f3e000
madvise(0x2b3b10f3e000, 1158680, MADV_SEQUENTIAL|0x1) = 0
mprotect(0x2b3b10f56000, 1044480, PROT_NONE) = 0
mmap(0x2b3b11055000, 16384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 4, 0x17000) = 0x2b3b11055000
close(4)                                = 0
(...)

----- out of memory case -----
(...)
open("/usr/local/perl/5.8.8/lib/site_perl/5.8.8/x86_64-linux/auto/DBD/Pg/Pg.so", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300_\0\0"..., 832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=93669, ...}) = 0
mmap(NULL, 1127840, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4, 0) = 0x2b7780b99000
madvise(0x2b7780b99000, 1127840, MADV_SEQUENTIAL|0x1) = 0
mprotect(0x2b7780baa000, 1048576, PROT_NONE) = 0
mmap(0x2b7780caa000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 4, 0x11000) = 0x2b7780caa000
close(4)                                = 0
open("/usr/local/postgresql-7.4.5/lib64/tls/x86_64/libpq.so.3", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/tls/libpq.so.3", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/x86_64/libpq.so.3", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/libpq.so.3", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0o\0\0\0"..., 832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=131049, ...}) = 0
mmap(NULL, 1158680, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4, 0) = -1 ENOMEM (Cannot allocate memory)
close(4)                                = 0
munmap(0x2b7780b99000, 1127840)         = 0
(...)