If you really need to do this, then go to the OS direct for the memory and you can guarantee to be able to give it back when you're done with it:

#! perl -slw
use 5.010;
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'FreeMem', CLEAN_AFTER_BUILD => 0;

struct s {
    int field1;
    int field2;
    int field3;
};

#define SIZE ((int)10e6 * sizeof( struct s ))

int test( SV *dummy ) {
    char buf[256];
    struct s *m;

    printf( "CheckMem then enter"); gets( buf );

    m= VirtualAlloc( NULL, SIZE, MEM_COMMIT, PAGE_READWRITE );
    if( !m ) croak( "Couldn't allocate virtual memory: %d\n", GetLastE
+rror() );

    printf( "CheckMem then enter"); gets( buf );

    VirtualFree( m, SIZE, MEM_DECOMMIT );
    printf( "CheckMem then enter"); gets( buf );

    return 1;
}


END_C

test( 1 );

__END__
C:\test>freeMem-IC.pl
CheckMem then enter   7.5MB
CheckMem then enter 122.1MB
CheckMem then enter   7.7MB
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