Every time push is forced to allocate more memory, it needs to copy the entire array. This can be avoided by preallocating enough memory.

my $count = @array;
$#array = $count*2 - 1;
for (1 .. $count) {
    push @array, $value if ($value = shift @array) !~ /^\#/
}
[download]

In terms of scalability,

• The grep solution you provided uses O(N) time and O(N) memory.
• The splice solution you provided uses O(N²) time and O(1) memory.
• The shift-push solution you provided uses O(N²) time and O(N) memory.
• The shift-push solution I provided uses O(N) time and O(N) memory.

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