>perl -wMstrict -le
"my $s = q{foo `bar baz` quux};
 my $t = $s;
 $t =~ s{ ` (\w+ \s+ [\w\s]+) ` }
        { (my $r = $1) =~ s{\s}{_}xmsg;  $r }xmsge;
 print $t;
 $t = $s;
 $t =~ s{ (` \w+ \s+ [\w\s]+ `) }
        { (my $r = $1) =~ s{\s}{_}xmsg;  $r }xmsge;
 print $t;
"
foo bar_baz quux
foo `bar_baz` quux