# if the current line is in $_
s/^|$/"/g;

# if the current line is in $line
$line =~ s/^|$/"/g;

# or with [doc://substr]
substr $_, 0, 0, '"';
substr $_, length $_, 0, '"';
# but it is simpler just to append a "
$_ .= '"';

# or even
$_ = '"' .$_ . '"';