my $line = ' ' x 200;
my $cols = {
    0    =>   \$var1,
    19   =>   \$var2,
    179  =>   \$var3,
    199  =>   \$var4
};
for (keys %$cols) {
    my $val = ${$cols->{$_}};
    substr $line, $_, length($val), $val;
}
print MERGE $line, $/;