Here's another, slightly different approach. It tries to factor regex elements, and to build an extraction regex (rather than a split pattern of some kind) based on a mapping of all valid commands to their possible values. I make some assumptions:

• an  $integer is zero or positive, never negative (update: and it never has a sign);
• certain real numbers like .0 or 0. need not be recognized;
• some commands like  N G have  $integer values only, and others like  X Y have only  $real values (the OP says a command is "a letter followed by a number which can be either an integer or real", implying that N, for instance, could have a real value);
• there is no inherent order in the occurrence of the  N G X Y etc. commands in a command sequence.

Note that the command sequence is validated before commands are extracted (the undefined  'C' command in one of the example sequences causes an abort). Also note that I use the  (?|pattern) extension of Perl 5.10+ (see Extended Patterns in perlre); if you need to avoid this construct, I can provide an alternative.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $integer = qr{ (?<! \d) \d+ (?! \d) }xms;
 my $real    = qr{ -? $integer [.] $integer }xms;
 ;;
 my %cmds = (
   N => $integer,  G => $integer,  H => $integer,  M => $integer,
   X => $real,     Y => $real,     I => $real,     J => $real,
   );
 ;;
 my ($rx_cmd) =
   map  qr{ (?| $_) }xms,
   join q{ | },
   map  qq{($_) ($cmds{$_})},
   keys %cmds
   ;
 ;;
 for my $s (qw(
   N260G02X142.05Y649.62I77.33J58.34H2M25
   N265M20
   N270G45
   N290G03X324.39Y638.4I-69.58J67.4H2M25
   N275G01X304.78Y608.8C45.91M25
   N280M20
   )) {
   print qq{seq: '$s'};
   die qq{bad: '$s'} unless $s =~ m{ \A $rx_cmd+ \z }xms;
   my %parts = $s =~ m{ $rx_cmd }xmsg;
   dd \%parts;
   }
"
seq: 'N260G02X142.05Y649.62I77.33J58.34H2M25'
{ G => "02", H => 2, I => 77.33, J => 58.34, M => 25, N => 260, X => 1
+42.05, Y => 649.62 }

seq: 'N265M20'
{ M => 20, N => 265 }

seq: 'N270G45'
{ G => 45, N => 270 }

seq: 'N290G03X324.39Y638.4I-69.58J67.4H2M25'
{ G => "03", H => 2, I => -69.58, J => 67.4, M => 25, N => 290, X => 3
+24.39, Y => 638.4 }

seq: 'N275G01X304.78Y608.8C45.91M25'
bad: 'N275G01X304.78Y608.8C45.91M25' at -e line 1.
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