$ echo '<ref id="ref1">
<ref id="ref2">
<ref id="ref3">
</end>
<ref id="ref4">
<ref id="ref5">
</end>
<ref id="ref6">
<ref id="ref7">
<ref id="ref8">
</end>' | perl -lne'
    if ( /<ref\s+id="([^"]+)">/ ) {
        push @x, $1
        }
    elsif ( /<\/end>/ ) {
        print "New Group:", map " $_", splice @x
        }
'
New Group: ref1 ref2 ref3
New Group: ref4 ref5
New Group: ref6 ref7 ref8