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my $input = 'hello';
my $temp  = 'world';
$input = "$input $temp";

my $input = 'hello';
my $temp  = 'world';
$input = $input . ' ' . $temp;

my $input = 'hello';
my $temp  = 'world';
$input .= ' ' . $temp;

my $input = 'hello';
my $temp  = 'world';
$input = join(' ', $input,$temp);

my $input = 'hello';
my $temp  = 'world';
...
push @temp, ' ';
push @temp, $_ for split('', $temp);
$input = join('',@temp);