There is probably a better way, but this is tested and does work.

I could get it to eval the value of $1 into $str either.

Using qr// to put your regex into a string save much bother of escaping thing and saves you from needing to eval the regex.

#! perl -sw
use strict;

my $exp= qr/^DMSC0022\s+\S+\s+(\w+)$/;
my $str='Logfile_Connection_Lost Hostname: ?';
$_="DMSC0022 X10 oswald";

if ( m/$exp/) {
    my $bit = $1;
    $str =~ s/\?/$bit/;
    print $str;
}
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