Your description of the input format is lacking.

If those numbers represent the numeric values of the bytes of a string, then this is just reconstructing that string from the information supplied:

my $bytes = pack 'C*', 0x00, 0x00, 0x01, 0x16, 0x7A, 0x53, 0x7C, 0x80;
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And that can be decoded as a 64-bit network-order unsigned integer using:

print unpack 'Q', scalar reverse $bytes;;
1196053200000
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If you have those numeric values in an array of scalars somewhere, you can skip a level:

print unpack 'Q', pack 'C8',  reverse 0x00, 0x00, 0x01, 0x16, 0x7A, 0x
+53, 0x7C, 0x80;;
1196053200000
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Update: And if you have 5.10 with the fancy new endian modifiers, then:

print unpack 'Q>', pack 'C8',  0x00, 0x00, 0x01, 0x16, 0x7A, 0x53, 0x7
+C, 0x80;;
1196053200000
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If you don't have a 64-bit integer capable perl, it takes a little more work:

( $hi, $lo ) = unpack 'NN', $bytes;;
print +($hi * 2**32 ) + $lo;;
1196053200000
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