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$string="abc de fgh ijkl";

...
{
    print "$1 $1\n";
}

abc de
fgh ijkl

abc de
de fgh
fgh ijkl

$string =~ /([a-z]+) ([a-z]+)\G(?: [a-z])*?/g

my @words =split $string;
for($i=0;$i<$#words;$i++)
{
   print "$words[$i] $words[$i+1]\n";
}