Perhaps something like this?

#!/usr/bin/perl

use strict;

local $/ = '}';
while (my $line = <DATA>) {
    $line =~ s/(fixed-address\s.*;)\s(hardware ethernet\s.*;)/$2\n$1/;
    print $line;
}

__DATA__
# Host 1
host 45583 {
filename "junk1.cm";
hardware ethernet 11:42:a3:d4:55:83;
fixed-address 10.100.34.114;
}
# Host 2
host D78C3 {
filename "junk5.cm";
fixed-address 10.100.34.117;
hardware ethernet 11:42:a3:FD:78:C3;
}
# Host 3
host 3A684 {
filename "junk6.cm";
fixed-address 10.100.34.119;
hardware ethernet 11:42:a3:13:a6:84;
}
# Host 4
host 46d54 {
filename "junk4.cm";
fixed-address 10.100.34.120;
hardware ethernet 23:10:3d:14:6d:54;
}
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