In situations like this, I find it a little more natural to shift off the elements, rather than iterate using a counter. But it means you have to take measures to keep from destroying the source, if that would matter. Putting it all in a subroutine can provide this measure of safety.

sub foo # not sure what you'd call this
{
    my $pat = shift;
    my @r;
    while (@_)
    {
        my $x = shift;
        push @r, $x;
        $x =~ /$pat/ and $r[-1] .= shift;
    }
    @r
}

my @new = foo( qr/string1/, @array );
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