perl 5.8 compatibility

[DrHyde]

On perl 5.8.9 there is currently a syntax error:

t/80extra_clauses.t ....... Useless use of unshift with no values at /Volumes/Vault/david/git-repos/sql-abstract/blib/lib/SQL/Abstract/Plugin/ExtraClauses.pm line 126.
syntax error at /Volumes/Vault/david/git-repos/sql-abstract/blib/lib/SQL/Abstract/Plugin/ExtraClauses.pm line 126, near "]{queries"
syntax error at /Volumes/Vault/david/git-repos/sql-abstract/blib/lib/SQL/Abstract/Plugin/ExtraClauses.pm line 126, near "},"
syntax error at /Volumes/Vault/david/git-repos/sql-abstract/blib/lib/SQL/Abstract/Plugin/ExtraClauses.pm line 128, near "}"
Compilation failed in require at /Volumes/Vault/david/git-repos/sql-abstract/blib/lib/SQL/Abstract.pm line 394.
t/80extra_clauses.t ....... Dubious, test returned 9 (wstat 2304, 0x900)

This trivial patch fixes it.

[coveralls]

Pull Request Test Coverage Report for Build 75

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 80.885%

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Totals
Change from base Build 73:	0.08%
Covered Lines:	1024
Relevant Lines:	1266

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💛 - Coveralls

[ilmari]

Sorry, I didn't see this PR, but I fixed it myself directly in master with commit c5dd2e6.