Speed up check for illegal top-level op

[larryl]

Calling List::Util::first here keeps showing up as a hotspot on our NYTProf output, so just use a regex instead.

[codecov-commenter]

Codecov Report

Merging #21 (85eb1ba) into master (7bed2f6) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 85eb1ba differs from pull request most recent head 8d3d33c. Consider uploading reports for the commit 8d3d33c to get more accurate results

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   74.57%   74.53%   -0.04%     
==========================================
  Files           7        7              
  Lines        1341     1339       -2     
  Branches      384      384              
==========================================
- Hits         1000      998       -2     
  Misses        252      252              
  Partials       89       89              

Impacted Files	Coverage Δ
lib/SQL/Abstract.pm	73.09% <ø> (-0.04%)	⬇️
lib/SQL/Abstract/Test.pm	71.42% <0.00%> (-0.29%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 76

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 80.791%

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Totals
Change from base Build 73:	-0.02%
Covered Lines:	1022
Relevant Lines:	1265

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💛 - Coveralls