because grep returns in scalar context the number of matches

Demo

  DB<13> @a= ('string') x 5

  DB<14> x @a
0  'string'
1  'string'
2  'string'
3  'string'
4  'string'
  DB<15> print @a == grep { 'string' eq $_ } @a
1                                                   # true
  DB<16> push @a, 'nope'

  DB<17> print @a == grep { 'string' eq $_ } @a
                                                    # false
  DB<18>
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