So string eval works so long as the entire trailing code block is in the string, which gets pretty clunky for one scenario I'm dealing with. But it works, which is better than I've gotten on my own.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my @data = (
    {unit   => 'S',
     value  => 50,
     },
    {unit   => 'T',
     value  => 60,
     },
    {unit   => 'Q',
     value  => 70,
     },
);

LOCAL_BLOCK: {
    my $cmd;
    $cmd .= "local \$data[$_]{unit} = 'S';\n" for 0 .. $#data;
    $cmd .= 'print Dumper \@data';
    eval $cmd;
}

print Dumper \@data;
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I'll do some digging on the cited modules, to see if they work in my context.