my @array = map {  substr $string, $_, 4  }  0  ..  length( $string ) 
+- 4;
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If the string is really long, use a $position variable and a while loop so that internally the big 0 .. length( $string ) - 4 list doesn't need to be built and stored:

my $pos = 0;
while( $pos <= length( $string ) - 4 ) {
  push @array, substr( $string, $pos++, 4 );
}
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