This may get you started.

#! perl -w

my $output = '';
while( my $line = <DATA> ) {
    # get rid of line terminator
    chomp $line;

    # Get rid of line numbers
    $line =~ s/^N[0-9]{2}\ //;

    # Replace * and the beginning of the line with
      #(Assumption: No further processing of comments required.)
    $line =~ s/^\*(.*)/!$1/ and print $line and next if $line;

    # Keep any numbers prefixed with X,Y or Z.
    $line =~ s/(X|Y|Z)([0-9.-]+)/ $2/g if $line;

    # Throw the rest away.
    $line =~ s/[A-Z][0-9.+-]+//g if $line;
    print $line;
}
__DATA__
N38 * TOOL 11: 0.125 DIA. FINISH ENDMILL
N39 M6T11
N40 E1
N41 S15000M3M7
N42 G17G90G0G8X-5.58Y-18.
N43 Z0.05H11
N44 G82G98X-5.58Y-18.Z-0.2R+0.01P50F100.
N45 X-3.171
N46 X-1.025
N47 X0.887
N48 X3.371Y-18.688
N49 Y-17.312
N50 X5.46Y-18.
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Output (asked for) followed by actual. I think that you made a mistake in what you asked for. Correct me if I am wrong.

! TOOL 11: 0.125 DIA. FINISH ENDMILL 5.58 -18. 0.05 -5.58 -18. -0.2 1.
+025 0.887 3.371 -18.688 -17.312 5.46 -18.
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(actual!)

C:\test>182489 
! TOOL 11: 0.125 DIA. FINISH ENDMILL -5.58 -18. 0.05 -5.58 -18. -0.2 -
+3.171 -1.025 0.887 3.371 -18.688 -17.312 5.46 -18.
[download]