Here's one way that reads in the string a little bit at a time, which is good if your data is very large. Mostly untested, but works for your data:

use warnings;
use strict;

my @wanted_substrings;

{
    local $/ = 'END';

    while(my $string = <DATA>) {
        push @wanted_substrings, ($string =~ /START(.*)END/);
    }

}

print "@wanted_substrings";
    

__DATA__
somecharSTARTfvENDsomecharSTARTsvENDsomecharSTARTtvENDsomechar
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