my @big = qw(a b c d e f g h i);
my @kw= qw(f c d g); 
my %small = map {$_=>undef} @kw;

exists $small{$_} or push @kw,$_ for @big; 

# or, Alternatively,
#    push @kw,grep {!exists$small{$_}}  @big;

print qq(@kw\n);

-- Output --
f c d g a b e h i