23+15=38 so "continuing on to position 28" doesn't make a lot of sense. Assuming that you meant (23,15) & (48,10), you could us unpack:

($1,$2) = unpack 'x23 A15 x10 a10', $var;
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If you insist on using a regex, then with the same change of spec as above, you could use:

$var =~ m[^.{23}(.{15}}.{10}(.{10})];
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Or if you really did mean overlapping bits then you could use substr

$1 = substr $var, 23, 15;
$2 = substr $var, 28, 10;
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($1,$2) = unpack 'x23 a15 X10 a10', $var;
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Or a regex:

$var =~ m[(?=.{23}(.{15}))(?=.{28}(.{10}))];
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Of course, using named vars rather than $1 & $2 would make more sense :)