$_ takes each value of **@B**, since it's grep {...} @B 
@A = (1, 4, 7, 10, 14);
@B = (1, 3, 2, 4);

@seen{@A} = ();
# does:
# $seen{ 1} = undef;
# $seen{ 4} = undef;
# $seen{ 7} = undef;
# $seen{10} = undef;
# $seen{14} = undef;

@C = grep { !exists($seen{$_}) } @B;
# In the grep evaluator, $_ takes the value of each
# element of argument list, one at a time.
#
# !exists($seen{1}) evaluates to false (undef).
# !exists($seen{3}) evaluates to true  (1).
# !exists($seen{2}) evaluates to true  (1).
# !exists($seen{4}) evaluates to false (undef).
#
# grep returns the values that resulted in true,
# so the result is:
# @C = (3, 2);

@merged = (@A, @C);
# @merged = (1, 4, 7, 10, 14, 3, 2);
#          @A---------------  ----@C

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In reply to Re^5: Merging Arrays without duplicates by ikegami
in thread Merging Arrays without duplicates by perl_krish

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