> does not (at least to my eyes) show what the difference is between them.

you are comparing

• perlre#(?^alupimnsx)

  "... for the remainder of the pattern or the remainder of the enclosing pattern group (if any)..."

• perlre#(?^aluimnsx:pattern)

  "... this is for clustering ..."

in your example there is no difference, but in the second approach with pattern the reach of modifiers is limited to the group.

  DB<24> p 'xX' =~ /(?i:X)X/
1
  DB<25> p 'xX' =~ /(?i:X)x/

  DB<26> p 'xX' =~ /(?i)Xx/
1
  DB<27>
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