You can use the rarely used third parameter to split to limit the number of elements extracted from the string:

$s = q[BONDING_MODULE_OPTS='mode=active-backup miimon=100 primary=p6p2
+ primary_reselect=1'];;

( $key, $value ) = split '=', $s, 2;;

print "<$key> <$value>";;
<BONDING_MODULE_OPTS> <'mode=active-backup miimon=100 primary=p6p2 pri
+mary_reselect=1'>
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Update: If you need to further split the value into a hash then you could do it this way:

$s = q[BONDING_MODULE_OPTS='mode=active-backup miimon=100 primary=p6p2
+ primary_reselect=1'];;

( $key, $value ) = split '=', $s, 2;;

print "<$key> <$value>";;
<BONDING_MODULE_OPTS> <'mode=active-backup miimon=100 primary=p6p2 pri
+mary_reselect=1'>

$h{$key} = { map{ split '=', $_ } split ' ', substr $value, 1, -1 };;

pp %h;;
(
  "BONDING_MODULE_OPTS",
  {
    miimon => 100,
    mode => "active-backup",
    primary => "p6p2",
    primary_reselect => 1,
  },
)
[download]